In short, let's say I have a column USERNAME in my table. I share my table with a collaborator called Julia. Julia can only see the rows where the column equals to USERNAME is "Julia". It's basically like a filter but the the collaborator cannot unfilter it. Julia would have only viewing / editing access to the above mentioned rows.
RLS are handy as they can scale. In our case, I might have 20 collaborators in the end. All collaborators should only be able to see their "own" rows.
I don't think such functionality exists I'm afraid. Perhaps you could get around it with an Interface that only displays and allows them to edit records that match the criteria? Not entirely sure that's possible
You'd end up having to manually create one interface per collaborator though I think
If you're alright with third party things, I think StackerHQ might work for your situation