You are basically trying to live in Thailand, without being a "tourist" in Thailand. They are just suggesting you apply for a long stay visa.

Assuming your passport allows (American, British, Australia etc) A tourist visa is good for 2 months right off the bat. You can also arrive without a visa at all and get 60 days exempt currently (this will probably go back to 30 days exemption sometime this year, so keep an eye on that).

Whatever country that wants to let you in. DTV is Thailand specific.

Depends on when and for how long you are coming for. In one or 2 months (I am not sure of the date), you will need to fill out the arrivals card online before arriving. Sometime later this year or early next year, you will need to get an ETA.

Also, that 60 day visa exemption (it is not a 60 day visa) may be shrinking back to 30 days soon.

Again, all depends on when you are coming.

No. Once you have left Thailand, you get a fresh 60 days when you go back to Thailand.

You answered your own question.

You need to get the appropriate visa to stay in Thailand long term.

I will assume you have done this more than once recently??

🤔

When you arrived in Thailand the last time, did they ask for, and did you have a departing flight and hotel accomodations (not ABnB etc) to show?