Yes, I turn mine off until I need it because otherwise you get junk calls and messages coming through as well and as soon as you answer a junk call the 100 mins start to go down
Just use Skype, or if your phone allows it download an amaysim Esim from Australia, put an $10 or whatever value international call package on it, and turn on international roaming via the app as you need it. Then turn it off, then you can also get messages from Australia even though it's turned off. I have one with 100mins now for 2 years, just turn it on and off as I need it
There is a racket called Doctor shopping, where people get get multiple scripts and sell at a profit to some poor sod in a third world country, thats the reason for the law
As others say, you probably can be Visa Exempt, you don't say what your nationality is, so you need to check according to your nationality, also given You are visiting several countries I would just buy travel insurance, or get a decent credit card that includes Insurance if the travel is booked using the card
As a frequent visitor with a Thai wife I use it every time, I always ask the Immi officer in Thai, how long is your shift, are you busy, and thank them as I leave. Im not a good Thai speaker, but I have those phrases memorised, usually saves queueing
As Stated elsewhere, the Thai Tax department is going to have to man up extensively to check all the people that are about to come into the system, it will take years to catch up. They will be concentrating on the big players who until now have kept their profits offshore for 12 months, then bringing them in tax free. The farangs living here are small fry in the scheme of things, and they are already getting VAT every time you spend a Baht. It will probably arrive as a deduction from bank accounts where no tax file number is provided.
If it's an EVisa, it will automatically pop up and be activated as soon as your passport is scanned, not sure if immigration will allow you to keep it for next time, I would ask the question before hopping on the plane