You're doing the 90 day report. It's free. If you miss it it's max 2000thb fine. Need copies of passport I'd page, copy visa.copy tm6. Should be straight forward. Once done in person (first occasion) should be able to do it online. (however has been playing up for some people)
mentioned, GoFund me retrospective insurance is quite popular . You do also hear of people on the bones of their arse, ill health and so on... but it's not that common.