Just use Git worktrees and a lightweight VM environment (I like macOS native sandbox-exec) and you can spawn as many sessions as you want. I've run upwards of 30 at once on my M2 Pro with no noticeable resource impact.
I would love to know more, but am so ignorant about working with multiple sessions running in parallel that I don't know where to start - do you have a blog or published article to share that might get people like me into a more educated space