Random advice: look into the Philippines, where you can live legally, practically forever, on a tourist visa.
As you were planning to work online on an ED visa in Thailand, which is illegal but with minimal risk of consequences, go do the same in the Philippines instead, where the visas will never be an issue.
I've been here 7 years. I have spent 30,000 baht in a month and I have spent 150,000 baht in a month.
If you're asking what it costs to just live here, look at rental prices in the area you want to stay in, expect to spend around 10,000 for food and 3,000 on electricity, add on a motorbike rental, taxi, internet, and medical insurance to see how much you will spend per month.
If it's just a few devices and you watch YouTube and chat, the lowest tier is enough. 20mbit is enough, but the lowest they have is 300, so get that one.
Having lived in both countries, I would recommend that you stay in PH and only visit Thailand for 3-6 months at a time.
Immigration here will give you a constant migraine, especially after you have had it easy with the immigration rules in PH. Also, the English levels among the population here are not even comparable to PH.
If you insist on coming here, Hua Hin and Chiang Mai are 2 cities that match your criteria.
It's hard for someone who doesn't already speak a tonal language since tone is the basis for the entire language. And since Thailand hasn't developed a universal way to write the tones with roman letters, there is no easy way to teach the language to big masses at an effective rate.