How many museums are you thinking about visiting? I found the Museumkaart was worth purchasing (if you visit the "Big 3" - e.g., The Rijksmuseum, Anne Frank Huis, VanGogh Museum - you have pretty much already covered the cost of the Museumkaart and everything after that is pretty much a freebie). The Museumkaart is good for a full year and it's great if you want to break up your time at some of the large museums into multiple visits over your trip. But frankly, no need to purchase it online ahead of time. Go online and see the different museums that are covered and pick a smaller museum you can stop by and purchase it directly from.
A lot of people recommend getting timed tickets for Anne Frank, but I went there for the first time during May when it was in the middle of tulip season and the city was busy. I saw lines during the day, but I went around 6:00PM and walked right in - no line at all. It's open later, so if you are able to wait until later in the evening, there is a good chance you won't really need a timed ticket.
The kaart will let you skip the ticket buying lines, but if there are security lines at some of the museums you have to go through those.
At popular restaurants, it really depends what time you are planning to eat dinner. If you go early, you may be able to get a table without booking far in advance - or if it is not some difficult to get into Michelin starred restaurant, you can probably just wait until you arrive and make some reservations a day or two in advance (particularly if you aren't talking about a weekend).
I found Amsterdam to be pretty compact and walkable with many major sights close together. When I did need to take public transit (e.g., trams, subway, bus) it was easy enough to either get a ticket on board from the driver or at the station or one of the main Tourist Info officers over near Centraal, and if you are not traveling solo, it can often be not that much more to simply grab a taxi.