This is actually a place I avoided in the past because of many reports of it being a mafia run island, which I've read on this forum. Plan B was Phuket, but that is another place I don't really fancy.
I need to take my truck with me just in case my wife needs oxygen. I will take a couple of bottles, just in case, but hopefully it won't be needed.
My very rough plan so far is:
Day 1 - drive to Surat Thani and stay the night. (any hotel recommendations please?)
Day 2 - drive to ferry terminal. I've been advised Sea Trans is the best of the bunch, and I see the car will cost 610฿ each way and passengers are 190฿ each way. (any comments on Sea Trans? I've looked at photos and it looks a bit like I.O.W. ferries. Best to book in advance or on the day?)
Days 2 to X (not sure of duration) - Check in to hotel. (any recommendations on area/hotels please? I'm not looking for beach life - I'll have the car and we'll move about/explore a bit - moans about wheelchair friendliness to follow

Final day - Breakfast, ferry back to mainland and drive home - overnight hotel somewhere if tired.
So, a very sketchy plan so far, so any help would be appreciated. Also on 'wheelchair friendly' things to see and do. I guess waterfalls will not be suitable - an able bodied cousin took a fall at a Samui waterfall in 2019 and had to be taken off the island by air ambulance.