To be fair this is all on the Thai Embassy websites in your country which you don't state
Tourist TR entey gets you 2 months 30 days + 30 days but you may also need a visa or get visa free entry depends on nationality/coming from where and must do Thailand Pass.
Special Tourist Visa gets you max 9 months. Lot of paperwork attached to this.
Try Google and the Thai Embassy site in your home country.....it is all there
anyone can get a residence certificate. It's just based on the reporting that your landlord or hotel do, telling immigration where you officially live. Most agents will get you one next day for 1500bht
in pre covid times I used to fly in on visa exempt every other weekend. Never an issue and once I asked and was told as long as you fly in and out each time (and not a bounce where you just come straight back) unlimited entries. But that is precovid....what the future will be is unclear