The biggest issue I have with dental work in Thailand is that it take sooo freaking long.
To have an implant takes 7-8 months and several visits. If/when you talk to dentist be sure to understand the whole time line. They'll tell you one thing but mean another so you need to be very specific... when will it all be done and the mouth 100% healed and you will not need more appointments. I think something gets lost in translation and they'll tell you 3 months... but what they mean is 3 months for one part, but you need other parts (extraction, heal, check-up, implant, heal, checkup, finish, heal check-up).
Don't expect to come for a week and have all your work done.
If you want shopping, you have to deal with tourist/foreigners. A majority of Thai folks don't have the money to buy bunches of useless junk. I would suggest shopping in Bangkok 2-3 days then off to the better spots.
I don't how crowed it is, but Koh Lipe is not easy to get to so I suspect there are fewer tourist.
Krabi is pretty nice (Ao Nang). The beaches will have loads of tourists but fewer that Patong (Phuket). Otherwise you will want to ask locals for the best spots.
You don't need a "home base" the TM 30 require is just when you have business at immigration. You don't have to go to the same immigration each time.
You can stay in Bangkok for a few weeks/months etc do your TM30, then go some other place and enjoy yourself. Next time you need to go to immigration make sure you have a current TM30 on file (this will be done for you by any normal hotel).
You'll need a bank account here in Thailand. Many people use Wise. There is a bit of a hiccup, in that you have to specific designate the funds as "international" when transferring to Thailand.
Nobody knows... The previous PM said... but nobody did anything and so far nobody has done anything, so nobody knows anything. Many people speculate but that is another story.
You're proposal is more expensive then simply getting the retirement visa from the get. However, there are several factors to a retirement visa and "your friend" would have get involved.