I was originally gonna say "try a clean install of Mojave", but then I thought... wait; that's probably what they did already, as I don't know if there's even a way to "downgrade" without an erase & install.
Anyway— I'm on Mojave w/ 10.5.1 and it's running clean as a whistle. I didn't mean to run 10.5.1... but I just bought a new SSD drive, did a clean-install, and then realized I didn't know how to get the older version from the App Store, so here we are. Thankfully, it's running solid for me. Environment is there & operating normally (just recently did a bunch of SysEx programming through it).