It is all speculation at the moment for both extension and re-entry. Currently there are no checks for re-entry as some people been in and out a few times.
Strictly speaking you need a Volunteer Visa and a Work Permit to do volunteer work in Thailand. These are very difficult to obtain due to misuse of these visas in the past by both agents and the visa holders. However, people do volunteer with no issue at places like dog shelters etc. Bangkok Community Help and others have a volunteer system. Organisations like this welcome help and do great work. Have a look for them on Facebook.
I agree it is very much overblown. This is mainly due to financial advisors who have suddenly become international tax experts and bedwetters who have been taken in by them. I have been tax resident in 4 countries the last 20 years or so. The Thai Tax form is the easiest although the company I worked for paid tax advisors to do al foreigners returns. U cannot the form changing much for 2024 from 2023 except about the year remitted monies were earned. As you say with DTT's the money owed will be minimal plus this change in taxation rules is aimed at wealthy Thai's. No small fry foreigners.
As Thailand signed CRS (Common Reporting Standards) in 2023 banks have become tighter in opening accounts. In a tax thread recently a bank branch asked for a TIN to open the account. If this becomes standard it might get tricky for many (Current account holders too). The TIN might not have to be for Thailand but it will have to be for somewhere. Banks having a TIN on an account makes their life much easier. Not saying this will happen but a provincial bank branch asking for a TIN came out of left field.
I am just throwing the cat among the pigeons here so take it as that. What happens if the Immigration Officer asks to see ongoing proof of the Soft Power courses etc upon re-entry? (I do not even know if he can see the category of course) 🙂 :-)