Split to Dubrovnik on Friday... help

Where are you flying from? Split to Krka is only an hour, but with jet lag it could be a bit of a slog.
Split to Mostar on the other hand, is only two hours, so I would consolidate a bit and not add a one night stay in Makarska. I'd put that extra night either on Korcula or with the first stay (giving your more time for Krka, Split, and area--lots of other day trip options and there is a cool thing I'd like to see with a car one day--Imotski's lakes).
Late May is a wonderful time to be in Croatia, but the sea will be quite chilly, probably not swimming weather yet.
You will not find Korcula untouristy, but having a car will allow you to easily explore the island.
What's the concern about Montenegro?

So, overall, looks great but I would try to eliminate one of the one night stays to be sure.

Well! That is some seat-of-your-pants traveling! Weather looks promising though. I don’t know that I can help add clarity but here’s a few thoughts: I think giving yourself two nights in Split at the start would be beneficial. There are some actual things to see there, and flexibility as far as excursions/activites. You’ve a lot of driving on the docket so start slow.

I don’t think anyone on this forum has actually been to Mlijet. There’s not too many ferries that stop there. I had plans to visit, but after some time on the coast I just had a hard time seeing how much better Mlijet could be versus the rest of the stunning coast. It’s kind of all “rugged coastline and crystal clear waters” everywhere, including Dubrovnik. I’d pocket those two nights for somewhere else. Maybe split between Mostar and Korcula, Korcula would be an easy choice.

Are accommodations driving you to Cavtat instead of Dubrovnik? It’s a lovely little town, and has a couple great little beaches (on the other side of the peninsula) but it is really small. Like, I think I’d appreciate staying there, but probably not three nights. Especially as you seem like really active sightseers. I think it makes a lot more sense if it’s your base for dipping into Montenegro though.

Valadelphia- We are flying from Dublin, Ireland, so although we'll be tired, there hopefully won't be a lot of jetlag. I'm unsure about Krka as it would be a large section of the day in the "wrong direction" but my wife likes the idea and it does look beautiful so it may be on the list. The reason I thought we wouldn't go direct from Split to Mostar in one day was that I'd heard there were many beautiful places along the coast worth visiting once you leave Split, e.g Omis, Makarska, Podgora... do you feel they could be missed, or just fitted in on a day trip to Mostar? Imotski's lakes I hadn't heard of, but they look beautiful, I would be tempted to try and see those on the route to Mostar.
I think I am unlikely to swim due to the water temperature, but my wife happily gets in the Irish sea without a wetsuit even in the winter, so I think she'll be splashing about in the water at any opportunity!
Our only concern about driving in Montenegro is that my wife had seen a few posts stating that people dive more recklessly and dangerously in Montenegro, and that has made her very concerned about us going in our hire car (she will be doing most of the driving), and she was therefore wondering if getting a tourbus would be "easier".

Okay good, no jet lag worries there (should not assume we're all in US)!
I am not sure how much you wish to drive in one day, but one night stays are such a drag on time so I try to avoid them if possible.

It's been many years since I was in Montenegro, but the drive down to the Kotor bay area should be fairly well trodden and fine. Some complain about the border check, but it's not peak of the season yet. Some of the mountain roads can be very windy, but I think you'll be fine.

Awrzesinski - yes we do tend to go for the seat of pants travelling option. It's a bad habit as we miss a lot of the "looking forward" part, and we end up stressing about itineraries days before we leave! I'm sure we will still have a lovely time though haha.
We have booked our first 2 nights near Split, but I'd be reluctant to stay much longer as we don't want the rest of the trip to feel more rushed than it might already be.

We aren't necessarily fixed on Mljet, we'd read it was very lush and beautiful and swimming in the lakes was possible, and I think that was very tempting to my wife! I would be tempted to have an extra day in Korcula though, just to feel a bit more settled.

The reason we considered Cavtat was that it wasn't far from Dubrovnik, is closer to Montenegro, and is close to the airport for our final night, where we'll have to get up at 4am. I guess I was thinking it might be a "cover all bases" stop at the end of the trip, but again, we're not tied to it, was just an initial thought at this stage.

Trogir is easy, very close to Split Airport, you can wander for as long or as short a time as
you want. 1 day in Split actually seems reasonable if you can park close by, seeing the
palace and wandering Old Town is enough.

Cavtat seems fine as a lodging option, but I doubt it's worth a full day of your itinerary
vs Dubrovnik. But the time it takes to get to Dubrovnik from Cavtat for your sightseeing
might negate that. I don't think you want to try to park in Dubrovnik and the bus
schedule is more sparse than one might anticipate. Maybe dump the car, stay somewhere
on the Ploce side, and take the airport bus on the last morning?

There is way too much to see, so you have to know that you're missing things and enjoy
what you are going to get to.

Yes I'll give some more consideration to the accommodation at the end of the trip, I just thought Cavtat might be good "middle ground" between Dubrovnik, Montenegro and the airport, that would allow us to book a few nights in a row as a 'base', but I'll have to look at how long it would take us to get to Dubrovnik, park outside and travel in, Vs public transport.

We've already had to come to terms with the fact that there will be so much we won't be able to see, it's a shame we don't have 3 weeks haha.

We are considering if perhaps we should stay in Korcula longer and just take the passenger ferry on one of the days to visit Mljet, maybe rent ebikes when we get there. The only issue with that is that we wouldn't get to see the likes of the Odysseus Cave etc, and the cost of the ebikes may be more than just driving the car there via Prapratno. Decisions decisions

We did the day ferry from korcula to Mijet a couple of years ago. We had a private tour arranged but most rented bikes ( not e-bikes). It pretty flat easy biking. Many enjoyed the lake swimming. I think a day would be sufficient. It’s not really car friendly IMHO