project tabs is not working for you?

Have you tried subprojects?

Yes, but since I'm dealing primarily with midi all I get is a blank reference file in the main project. Midi doesn't pass from the subproject to the main.

Yes I agree - multiple timelines would be great. Also the possibility to start many times with bar 1 at different times and different tempos, while changing earlier tempos wouldn't affect the position and start of the other tempos... for scoring this would be amazing

I would look into virtual midi drivers and see if you can’t get the sub projects to play together as you want. If not, try either multiple portable installs and/or separate instances of reaper (via the action “spawn a new instance of reaper”). You could then have separate midi time lines in different instances or perhaps sub projects of one instance, and a separate install that is just always running with all of your instruments. There’s a lot options.