Overall, I think your workflow is not too dissimilar to the UX we are
proposing :)

[off-list ref] writes:

4. If working on the submodule, use a branch, not a commit - typically off main.

With the proposed UX, step (4) would happen automatically when using
"branch --recurse-submodules". Users would get a safer and more
convenient default.

What I could see as a possible improvement is to add the branch ref to the submodule ref file - not replacing the commit but adding to it. I do worry that there are unintended (unforeseen) side-effects that will result from this, however, including potential merge conflicts. Two people working on the same commit but different branches may mess the ref file, so not really a good idea.

It's an interesting idea, but as you noted, it is quite thorny. I would
also like to see more information being captured by the superproject
tree (instead of just .gitmodules), but I'm also not sure how we might
do that.

Just my musings.

I appreciate the effort taken :) Thanks!