Thanks for providing some updates, @Alex Kilby,
It looks like you've already tried all possible troubleshooting to fix this unresponsive error in QuickBooks Self-Employed billing page. Since trying a supported browser and opening a private window did not fix the problem, I recommend contacting our Support Team.
Our representatives can report this green spinning circle to our engineers and can help update your billing information for you. To contact us, click the following article: Contact QuickBooks Self-Employed Support
Let me know how the call goes. I want to make sure this concern is taken care of. Have a good one!
