Mojave VM is better than Big Sur. I use the Mojave VM for Notes, Calendar, Reminders, Numbers and Growly Notes. It works well for those programs on a fast machine. It works well on Big Sur except for shrinking windows. It takes four seconds to shrink a window (down from seven before I turned off animations). The host machine reports high CPU usage as does the guest machine.
I could just stay on Big Sur and never shrink a window but I just went back to Mojave instead. I prefer the look of Big Sur but don't like performance issues. The whole VM froze when shrinking a window. Bringing it back wasn't a problem.
I could use email and browser but the drawing is a tad slow. For some things, using VNC into a real Mac has better performance but I've found that you lose cut and paste with VNC and macOS Screen Sharing.