Read in official sites what the Thai Revenue leaders have said and stop spreading misinformation. I won't give you the links or you'd just accuse me of wanting clicks. Seek it out for yourself
Thailand is considering taxing the global income of those who spend more than 180 days here in any year. But right now the law is interested only in money you bring in to Thailand wherever it is earned. It doesn't matter whether the money is salary, pension, investment income, capital gains ... if you bring money to Thailand and stay 180 days, that money is assessable for Thai income tax. That said, tax already paid in another country will off-set tax assessed here, if that country has an DTA with Thailand
I have a very comfy 1-bed condo in Bangsaray, about 40 sqM. 20-25k is a big lump out of a UK pension, but of course will be much bigger in Pattaya or Jomtien
Rent, don't buy. Much more flexible if you want to move. No problems getting your money out if you relocate again (to another country, or heaven or hell!) About 8-10k baht a month rent, £180-220
If you stay 180 days or more, you're a tax resident. Cumulative in any calendar year, so a border bounce on day 179 doesn't help you. You (should) declare all the money you import to Thailand. It is assessible for tax. You show tax certificates in that tax year from home country, which is deducted from tax due in Thailand. Maybe you will be able to demonstrate that you transferred savings rather than income, but the mechanics for doing that are not clear so far. Maybe it would be as simple as showing a statement showing the funds leaving your home savings account.