Some itinerary help, please

You have too much on your first day - Leiden is great, so explore Leiden. See if you can arrange to pay for a half day and check into your hotel early, and grab a nap. Being a first time traveler, you have no idea how jet lag will affect you. I love Haarlem, but it's too much.
At Keukenhof, forget the bikes. We found them to be in terrible shape, so difficult to steer and avoid canals and opposite traffic. Bring a hearty lunch, as the lines for food are long.
Reduce the day trips overall. You can't soak up the culture if you're wizzing by it.
We had a wonderful time time this past April, so I wish you an equally fun adventure. Safe travels!

Is there a reason you're not staying at just one hotel? (Is that the Marriot certificate?) I just don't love switching and packing so my choice would be sleep in Leiden or Amsterdam and take day trips.

I'd encourage you to explore the art museum in Leiden. I really enjoyed it.

Pat and pbscd hit the nail on the head. Reduce the day trips, don't try to do too much on your first day, explore Leiden (which is one of my favorite places), and stay in one place. I would choose Leiden as a home base, as it's incredibly easy to get to all of your potential destinations from there. The Netherlands is one of the easiest places to do this. You can be in most of these places within 30 minutes and sometimes less.

And while it does take time to commute to these places (although not much), it doesn't take near as much time as it does to check into a new hotel every single day. I would hate that. It's much more relaxing to stay in one place and then go out and visit other places once you've had some time to explore where you're staying

If I read your message correctly , you arrive on a Monday (“ I will actually land on Monday”).
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Mondays are not great to visit Haarlem as most museums are closed and shops open after 13:00. The Monday market is just a couple of stalls and will not give you that experience you would be expecting l/hoping for

Mid April, changes are there will be the traditional fun fair on the Grote Markt. Although a local tradition that goes back over more than 100 years, it confuses quite a few visitors to see a Ferris wheel next to the gothic church. Maybe not what you are looking for after a long haul flight.

Combined with jetlag and being in a new place, I support the suggestion made by the others to go straight to Leiden and relax&adjust.

I'd pare down and break your itinerary up into only TWO hotels - one in Leiden and one in Amsterdam.

Mon (arrive AM)
- head to Leiden, drop bags/check in at hotel, grab lunch, get in a walk around Leiden or relax in a park/canal-side to fight jetlag, have an early dinner, and get a good night's sleep. Consider renting a bike in the afternoon (multi-day rental) at EasyFiets.

Tues (Keukenhof day)
If you have a bike rented, you can take the easy and scenic ride to Keukenhof. Spend the morning there wandering the gardens. Afterwards, hop on the bike and head to the sea and the dunes for a lovely time in nature, then loop back down towards Nordwijk and then back inland to Leiden. Spend the late-afternoon and evening enjoying Leiden.

Wed (Delft and ???)
Take the train (40 min) or ride your bike (1h 10min) down to Delft, and wander this lovely town. Grab a meal/beer canal-side, and check out some of the fun stores as well as the titling church and the other neat architecture. Maybe the Royal Delft museum if that interests you. Along the train route to/from Delft is The Hague which is has plenty to see and do even if you only want to spend a brief bit of time there. The Mauritshuis is a great and easy museum. Just past Delft is Rotterdam - a really neat modern city with a totally different vibe than many other Dutch cities. Even Gouda is "nearby", so a longish day of Delft, brief visit to Rotterdam, and afternoon Gouda is "doable" (a few ~45 min transit trips between each).

Thurs (transition day Leiden -> Amsterdam)
- Check out of Leiden hotel (return bike if rented), head to Amsterdam to drop bags/check in, and get familiar with your surroundings. Like Leiden, you could pop over to rent a bike at A-Bikes (one block from the Marriott), and head off to Vondelpark for a big loop through it, with an option for some food inside the park. Definitely consider a canal cruise in the evening one day here. A RS walking tour or two would work, but since they're relatively short, consider a meal/snack or two and some shopping too (towards end of Jordaan walk is good for this).

Fri (Rijksmuseum and ...)
- Most museums are best done at the earliest times or closer to closing times. I'd hit the Rijksmuseum early, and have the whole afternoon for other things. With a bike, you could spend the afternoon heading out toward Broek-in-Waterland, Edam, Volendam, and Marken. April will not have the super-long days of Summer, but you will still have 6+ hours to be out and about. For us, one loop back into Amsterdam from that area took us by Brouwerij 't IJ so we naturally stopped for beers and bar food before heading back to the hotel.

Sat (Dutch Resistance museum/???)
- we grouped the Dutch Resistance museum with the Anne Frank House and the Corrie Ten Boom house (in Haarlem), and it was a very good grouping. I'd choose the Anne Frank house (with a Diary reread ahead of time) as top of the three, but the other two really helped provide a broader view of WWII-era Amsterdam (and the Netherlands). IOW, I'd make sure I secured AF tickets, and build my time around that first, but I would also add the DR museum (only an hour or two visit) on one of these Amsterdam days. The DR is by both the Hortus Botanicus and the zoo (can see a bit from outside), so you could add the HB to the mix (haven't been there so not sure what or how long). We actually paired the DR museum with a ride over to the NEMO Science museum - not for a visit to the NEMO museum, but rather to access the ROOFTOP (free) of the museum for neat views of Amsterdam (and bathrooms and food options). Really neat place and probably a cool museum judging by how neat the rooftop was.

Sun (more walks/exploring options)
- with bikes, you can do more exploring the surroundings of Amsterdam. This could be riding through neighborhoods (way less hectic than the core of Amsterdam), heading out to somewhere "different". We've done loops down to Ouderkerk aan de Amstel, out through Westerpark and to the Sloterpas, up to the Zaanse Schans windmills, and out to Haarlem for the CB house. IOW, there are lots of options to ride and explore.

• on foot and via transit, there are plenty of options including your Hoorn and Zuiderzee and Flevoland or North-Holland. Or, just "more Amsterdam" exploring- plenty of shops, restaurants, museums, and parks to see and explore. If you didn't ride out to Broek-in-Waterland, Edam, etc., it still might be fun to hop the free ferry behind the Centraal station to head to the city on the north side of the river. A free boat ride is always a win in my book.

And we really like having a tasty rijsttafel meal, but as a solo traveler, maybe not ideal. I'd be interested in if others found it "fine" for solo folks, because I think it is a great meal which we generally don't have as an option here in the States. These is a lot of really tasty food in Amsterdam (and the Netherlands) so we always prioritize food when there.

Mon (depart AM)
- Not really sure how things will be at AMS in April, but for us, AMS has consistently been one of our least favorite airports to fly out of due to check in and security. Last time (pre-EES), we were stuck in security for 30+ mins - not moving an inch - because some moron a few people ahead of us put a "tourist" dagger in their carry-on bag. For whatever reason, security seemed incapable of isolating that person without stopping all scanning. Weird bad luck for us, but not our first frustration with AMS either.

IOW - give yourself a little more time than you might generally feel comfortable with when heading to AMS.

Mid-April much is already in full blossom and if the wind is not too strong a good opportunity to explore the area / countryside north of Amsterdam by bike or e-bike. Weather can still be chilly in April.

Depending where you rent a bike you can start for instance with Durgerdam, then Ransdorp, Holysloot, Zuiderwoude etc. You can go further to Marken and take the ferry there to Volendam.
The latter easy to combine with Edam.

On the way back you can visit Monnickendam and Broek in Waterland.

Another option is biking along the Vecht river south-east of Amsterdam. It’s well known for it’s 17th / 18th century country estates.

You can take the train to Breukelen, rent a bike there or if you have already one take it with you on the train. The best part is between Vreeland and Slot Zuylen (Castle) near Utrecht. Or you can start in Weesp and bike all the way to the latter and take in Utrecht the train back to Amsterdam for instance.

If having anough time you can combine this daytrip with Kasteel de Haar / De Haar Castle. Needs for instance a detour between Slot Zuylen and Breukelen.

Bikes on a train need seperate tickets and are only allowed during off-peak hours. Many train stations offer rental bikes, but think more suited for covering short distances.

https://www.ns.nl/en/travel/on-the-train/bikes-on-the-train