If you squash into a single commit upon merge, ignoring for the moment the fact that as a blanket rule that's a bad pattern, you've now eliminated one of the core arguments against rebasing. The merge commit adds no value if the branch itself is a single commit. Just rebase your squashed-into-one-commit branch ontop of latest master and push that to master instead. Now you have one commit representing your whole PR, with no pointless merge commit.

I really discourage the squashing upon merge approach entirely though, because that's just a bandaid for lazy and/or misinformed developers to cover up the fact that their whole git workflow is completely borked.