I'm happy that you enjoyed the compositions and vibe!
Like maybe one idea I had is that I might be relying too much on the overall mix reverb to do most of the work. But I think tweaking the reverb (or just any post processing in general) for individual instruments a bit more closely so I can get the mix that I want would have a greater effect in making the individual instruments more clear.
Paying attention to the overtones/panning is a good idea too. I'll look into multiband compression; I don't use reaper but it sounds like it might be a good idea to learn what a bit more of what some of these plugins do in my own DAW anyway. I know some of the plugins but there are probably hundreds and mixing is quite a bit of a different skill than composing.
Will be sure to keep in mind that some scenes need a bit more variation, I agree with your brain; I did have a small string ensemble playing a simple motif/counterpoint in the background but it sounds like I needed something a bit more obvious and upfront here, especially since it is the final scene and final part, gotta finish strong.
Thank you for your detailed feedback, I hope my feedback was useful for your piece as well.