you cant squash like that, as it cant track the commits. i prefer to generally, work in a branch or fork, and keep rebasing my commits into a clean commit and force push until an upstream is ready to accept the PR. this keeps out conflicts and makes PRs easy, but it ruins others being able to work with your branch (because of the force push).

you might be interested in 'git flow' workflow. i dont like it that much but i also dont tend to have long running feature branches that multiple people commit into.