I haven't prices them lately. My wife's AIS unlimited internet is $20 usd /month. My unlimited 5G is $30 a month. I think similar packages in the US will cost $$$$, so 'expensive' is relative.
I couldn't speak to Koh Phangan. I have never been there.
I was on one side of Koh Sumai last weekend. Every where I went the internet was pretty good. Occasionally I would find myself in a building on the side of the hill, which would get shotty internet, but mostly the internet was good.
Koh Tao had good internet on half of the island. The other half of the island was sparsely populated so there weren't many towers, no internet. If you stay were people live the internet was quite good. This was about 2 years ago.
I would choose AIS over DTAC. When DTAC works, it works pretty well. AIS seems to work many places. Can't buy AIS from 711 (or at least you can't pay your bill there). Maybe you can find AIS at the airport before Koh Samui ( assume it will be Thailand). AIS services several countries.
I would recommend Thai bureaucracy. You're going to take a hit on your stuff either way. You may discover you dont miss or dont need, some of your stuff and you may enjoy newer stuff. Thailand has much better things to do than watch TV.