I'd say git submodules have such an awkward UX that should probably not be used except in very rare and organized cases. I've done it before but it has to be worth it.
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.
But I get your larger point.