You can get paid from work done before you moved. But once you move any additional work would need a permit.
Proper way would be to setup a Thai company and get a work permit via that. Not sure what your original country of origin is but if US check out the treaty of amity, it gets rid of a lot of the downsides of having a Thai company as a non-thai.
And yes patching your product would count as work. Good things there are no bugs or issues with your product.
DTV wouldn't let you work in Thailand either. It lets you work remote for a company outside of the country not in country.
Several people have had the same issue of not planning far enough ahead and actually having their visa before their travel date. What you are talking about is what they have been doing waiting in country until they get approval then border bounce to switch visas.
The risk is in the embassy wants you to come in for some reason you are SOL.
But since you are already in a different country it isn’t that much extra risk.
ETA has been pushed back. The digital TM7 arrival form is different. The TM6 was what was supposed to come back on May 1st, but no official details are out about it yet.
Worse case you will fill it out on your phone while waiting in line at immigration. But if you are this worried about filling out a form you might want yo rethink Thailand. They love to make statements that go nowhere and change the rules constantly.
Just relax and roll with the changes or you will stress yourself out of any enjoyment.
They aren't a business though. They work for the airport which is semi government owned and run. When has anything remotely government been worried about being efficient or making money.