showing your 800k seasoned for 2 months in Thai bank, long term lease agreement. Go to immigration and get their document checklist. They can vary slightly. No need for an agent.
What is your non-o based on? Marriage may have hone visits and lots of documentation requirements, but retirement has mostly residence document requirements.
Sadly it will only get more restrictive. Abuses of the past (all nationalities) have essentially locked the doors for citizens of poorer countries. Easiest and quickest (3-6 months) is usually the fiancee visa from what I've read.
you might have to ask specifically for the letter. Bkk wanted 5 days to do the statements. When u showed up to collect there was no letter. I asked about it and if course it was another ฿100! 😁