Just consider that 800k baht as the price of living in Thailand on a retirement visa. I'd say it's worth it in terms of cost of living/quality of life.

I was in a similar situation, I tried to get a residence certificate at immigration but the Airbnb I was staying in gave me a TM30 that wasn't valid (something about the Italian owner of the condo not being on the title deed). K-Bank wouldn't let me open an account without it so I had to sign a one year lease at a condo. After I did that I was able to open an account at K-bank with the lease. When I applied for my one year extension the immigration officer made me also get a copy of the condo title deed (Tabien Baan) - this was the only thing I didn't have and didn't know I needed even though I researched a ton. Luckily the title deed was in the condo so I just made a copy of it and went back to immigration the next day and was able to get my one year extension.