git worktrees are a horrible misfeature; it's better to just clone multiple times (which you can do locally---space is saved by use of hard links!)

So that is to say, we clone some remote upstream down to /path/A. Then we can clone file:///path/A to /path/B locally. Both A and B are fully fleded git repos; no monkeying around with worktrees. The objects are shared between them with hard links. You can edit B/.gitconfig to point it to have the same upstream as A for pushing.

Unlike worktrees, you can blow away a repo with "rm -rf", and not worry that you are pulling the rug from under something which depends on it. If I have several trees, A, B, C, one of which is the parent, while the other are worktrees, removing any one of them randomly is Russian roulette: if we do "rm -rf B" and that happens to be the parent repo, worktrees A and C are no longer functional and need to be recovered. If A, B, and C are independent clones (even with objects hardlinked among themselves), this isn't a problem.

What you described sounds like this: https://github.com/GenseeAI/gensee-crate. It offers whole workspace fork and merge. But the merge is still somewhat cumbersome and require manual overseeing.