Doable 3 week itinerary for Croatia heading North?

Personally, I hate one-night stops, and even two-nighters make me sort of twitchy. But other folks don't seem to mind never settling in anywhere. I would at least combine the Bled and Ljubljana stops, though, and spend both nights in Ljubljana.

I'd like to suggest that you give some thought to what you mean by "see as much as we can". When you move around a lot, much of what you see is through a car or train window. You're likely to see more if you slow down.

I would do only one night in Mostar. It's worth an overnight one night, but it's not a big town (at least the old town you would likely be most interested in). Two nights in Mostar is probably too many unless you have some special interest there more than the average tourist would.

I'd spend the two nights in Split, not Zadar.

I'd probably take the extra night from Mostar and add it to Slovenia. Two nights in Ljubljana, one in Bled. Or three nights in Ljubljana with a day trip to Bled. FYI, you can get a direct train from Zagreb to Bled Lesce if you want and shorten the train ride from Ljubljana to Budapest a little.

I'd rent a car in Slovenia too, at least for a couple of nights, just to allow for some pretty scenic drives e.g. the drive between Bled and the lovely town of Skofja Loka, through the mountains via Jamnik and Kropa - should be spectacularly scenic sometime in October with fall colors, maybe while you are there. I guess you might push up against starting winter weather, not sure how soon the snows might start. But if you take the train to Bled, you could rent the car as you arrive and drop it in Ljubljana when you get there - you probably won't want a car there. You don't need to rent a car at all just to get between Ljubljana and Bled, but as I said, the scenic drives are pretty in that area, and it's worth stopping a few hours in Skofja Loka, too.

I'd spend even less time in Mostar - it's a lunch/afternoon stop. It's just really tiny. The bridge is cool. Stop on the way from Dubrovnik to Split. Use the extra night in Zagreb. I'd do Bled as a day trip from Ljubljana. I'd take Split over Zadar - historically its more important.
I'd take one night from Prague and put it into Ljubljana or Zagreb.

Another vote for more than just a partial day in Zagreb, if you can spare the time. The city has lots of interesting museums and a large historic district. It feels different from Ljubljana.

Zagreb is a gritty working city with a great university (U Zagreb), a very nice Cathedral and associated cardinal with an interesting story about the communist time, an art scene that is worth learning about (Croatian naive art), and several good museums. It is walkable. It has a lot of inexpensive places to stay. It has a Botanic Garden, that is closed more often than it should be. It has a cemetary that we need to visit next time we are in town. In short, worth at least 1 night and 2 solid days of sightseeing.

Ljubljana is also a working town (both are capital cities). It is a bit more touristy. It is also walkable, and has many nice restaurants. It has a great castle, with the advantage of a resident dragon.

Both are worth 2 days each.

I'd have described Zagreb as "sprawling" rather than "gritty", but I guess both are apt. If you're in the museum district or the adjoining old town, I don't think there's anything gritty to be seen. You will go through some characterless districts on the way to the good stuff, for sure.