@Robert ******
has sorted me out via Direct Message. A few takeaways:
1) need to make clear on this page what type of visa you are on. I was on an extension of stay, not simply a non-imm o visa. The details are vital to get the accurate advice you seek.
2) As it's my first report, I will need to go in person.
3) Online applications need to be made between 15-7 days before the report date.
4) Can only report in Bangkok at Chaeng Wattana immigration office.
Thanks for your time!