It was solved, I still don’t know why it creates an empty repository or quits, but I ended up asking someone else in my lab to try and uploaded the experiment files to a drive and they were able to sync it.
At first they got an error related to git, so they deleted the hidden git files (as suggested here Failing to push files to Pavlovia after removing an experiment from GitHub).
Then, when they pushed sync it seemed like it was frozen, but let it run for a while (ignored it overnight) and in the morning it said it was completed and the files were up in Pavlovia! My experiment still gets stuck on “initialising” so I have some more work to do!