Sure. Download the branch and use git.

   git checkout main
   git merge --squash pr_branch
   git commit -m "Commit message"

This doesn't work if you have branch protection rules blocking pushes to main, which in my opinion should be standard on any repo

Oh I forgot about that. I mostly work on private repo (free account) so that feature isn't available to me.

Nope. Pushing to main is only disallowed by default if you change history. A normal squash or rebase merge is allowed and should be allowed.

UI-only workflows are for dummies, they won't fly for larger projects

Define large projects. While you can use the cli Google has ui only workflows. I'm fairly certain Google probably has the largest project out there.

Didn't know that Google is for dummies. Matches my interview experience there