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.
"on arrival" I am not sure. Someone will answer that question.. However, you will want to have a hard copy of your visa whenever you travel in/out of Thailand or whenever you do business at immigration. Or at least that has been the case for every eVisa I have gotten.