come on visa exempt or 60 day tourist visa. Open thai bank account. Put ฿800K in it. Apply for 90 day retirement extension. Many threads about this. You will need insurance to enter Thailand, but not for this extension. Before 90 days apply for 1 year extension. THEN you show its been in account for 90 days.
You may have an overstayed stamp on your passport if you pay the final leaving. It may even affect your ability to get into other countries. I would’ve paid the extra 2500.
It appears that there are two online systems. Yes, the new, public – facing system replace the old one. But it seem fair is another one of the agents have access to. When you go in person or post a 90 day report, they enter it into the back office system, but apparently not into the public system. I had this trouble too. It didn’t show I was on over stay, but only my reports cervical times back when the old system is working. I don’t want to go to Muang Than Thani, so i have Bumrungrad do it. If you go there, they call their contacts at immigration.