Philip Connor . . . it is rather the other way around: in order to apply for the "change of visa type" from a tourist visa to a "90-days Non-Imm-O retirement visa" on Immigration, a one-year lease if very helpful, because a rental contract is one of the requirements for this application (here, Chiang Mai immigration)
there might not be any agents in Supan Buri, because this is not a very touristic area or an area used by many Expats. I recommend you take accomodation in either Hua Hin, Pattaya, or Bangkok, and proceed from there. After you got issued your 1-year Extended Stay Permit, you can register your accomodation in Supan Buri and use their Immigration for all future extensions
. . and then, from a certain age on. there will be another tax exempt sum added to the 150.000.- THB which are tax exempt any way. Plus deductible are children and elderly care. However you would need a tax lawyer to do the declaration for you, as up to now, no English text exists!
I thought the first 150.000.- THB are tax free, not 120.000.- THB like you wrote. And agreed to the rest. If I prove the monies have been earned before 31.12.2023, I guess they are all classed as savings and not as income, and remain tax free