long overdue update on an old mac

Hi, I've recently been having a number of problems with my version of Musescore (3.6.2). I checked the website, and saw that the newest version requires macOS 11.5 or higher. Since I'm on macOS 10.13.6, and can't seem to install any software updates on my end, would you be able to tell me

1.) Which version is the most recent that's compatible with my computer?
2.) Will I be able to migrate larger scores?

OR

How can I fix some of the problems* I've been having?

Thanks for your help! I can attach the file if necessary, but it's quite large.

*one certain part of the score causes playback to stop sometimes. Other times it's perfectly fine, and more rarely the muted trumpets will continue to hold their chord for a bar while the rest of the playback stops. Following this playback either continues as normal or stops, and can be resumed as if I had paused myself, or the program crashes.

*the program crashes randomly every once in a while, but not often enough to be an issue on its own.

*Sound issue: certain unisons (most noticeable to me in strings, clarinets, and horns) sound incredibly strange. The notes sound like they're jumping around in pan, one note at a time, sound entirely different in timbre note to note, sound much quieter, or much louder, or an octave or two higher, or not at all. I tried adjusting the intonation by a fraction of a cent to see if it was overtones doing weird things to the playback, but no dice. These audio glitches are present in audio files that I export