Hey man, I have composed for a casual game before for a client, you can search for it on Steam as "Goosey Guess", it's a guessing simulator. Our answer to the problem of keeping the player engaged through music was to compose long tracks with lots of variations and repetitions as well, it seemed to work, there were twitch streamers saying it was good and relaxing.
So, I think that's about experimentation, my example of the boss theme there is indeed an oddballs case, probably likeminded Dark Fantasy RPG or JRPG devs might be using it, but it's definitely not a standard approach.
But it's like what you said, it's difficult to find loops out there from different composers that will combine nicely (specially if you're looking through free assets it seems), hence my suggestion for composers to experiment creating different types of music packs!