What are the guidelines for the 90-day check-in for Thai visa holders, especially for first-time applicants?

After your first report you can do it online. The window is 15 to 7 days before your due date.

I do mine about 10 days early.

Record the details exactly as you input them and then next time you can simply copy the information to save you time.

the first 90 day report when you come back into the country OR when it's your first one has to be done in person.

You can try online, but most people are getting rejected with the notice that "first report done in person" message

Why leave it as late as possible?

No need to be a "day counter" on 90 day reporting. Your next report will be due 90 days from the date you do this one.. You don't need to max out the time between them.

You can apply in person from 2 weeks before the due date down to 1 week after the due date.