I can't get over my fear of subtrees after accidentally nuking one of my repos by doing a rebase across the subtree commit. I've found that using worktrees, with a script in the main branch to set up the worktrees, works pretty well to split history across multiple branches, like what you might want in a monorepo.

Sadly doing a monorepo this way with pnpm doesn't work, since pnpm doesn't enforce package version requirements inside of a pnpm workspace. And it doesn't record installed version information for linked packages either.