Apart from your financial needs for a permission to stay – i.e. visa and extensions of stay – yes, you can live a modest life in Thailand if your own your home and has a simple life-style, which still can be a good life due to weather conditions etc.
If you can cook yourself and don't eat huge amounts of food, you can live for 200 baht a day. Even eating out is possible for a few hundred baht a day , if you choose local food from street kitchens, markets or modest restaurants.
Me, I only eat one major meal a day and make an excellent mainly Western-style supper for in average 200 baht per day including a glass of wine or two, and some fruit or a bisquit during the day...😋 I could easily afford (much) more, but I don't need to, I'm happy with my life-style and rather spend my extra money on something else.
If you find the right area, where you don't need aircon – or much aircon – and avoid real cold periods, your electric bill can be less than 1,000 baht per month, less if you don't cook meals yourself usin electric power.
Your major concern – if you own a home and have modest expenses for that – is health. Healt insurance can be quite expensive depending of age and level of coverage. If your health is good – I would say: That is a must for a 20k baht budget – and you can settle with public hospital service, you might find a health insurance in the level around 3,000 baht to 4,000 baht per month.
So, to break down some kind of monthly budget:
6,000 baht for 200 baht per day food
1,000 baht for electricity
1,000 baht for communication (Internet/mobile)
4,000 baht for health (either insurance or savings for self-insured)
That leaves you with 8,000 baht for home fees/maintenance and other stuff you need or now and then dining out in a little more expensive place, to enjoy a happy life...