Comment by Kalq

1.projectile-kill-buffers or the built-in project-kill-buffers will do that for the current project. You could run that before switching projects.

2. https://github.com/radian-software/apheleia

3. There might be a way to do this but I'm not sure. Emacs being inherently single threaded probably makes this difficult. But yes, I use M-x (re)compile.

4. libvterm is the best still imo. You can definitely do multiple instances and there's even different tab modes in Emacs now.