Does the first 90-day reporting after re-entering Thailand need to be done in person or can it be done online?

As per Brandon, after re-entry into Thailand, first report is always physical.

I would suggest the same and proof it yourself .....report online 15-8days before due if it fails den do it in person

The official rule is that you must go in person for the first time on every entry into Thailand.

That being said, there are a few reports of people being successful when trying online report. Because the window to do your report starts at 15 days before your due date, there's no reason you shouldn't at least try to file online. Then if it's rejected, you still have time to go in person.