The fact you didn't submit tax returns, are 31, and single aren't going to see a tourist visa rejected. Most likely it's because you didn't submit flight details or didn't have sufficient money on your bank statements.
I don't know if they're issuing non-ED visas outside of Thailand in order to learn Thai except at a university maybe. I don't think any of the language schools are offering online classes anymore which will provide you with a non-ED visa, if they were then you can study it online in your own country.
as someone who is currently in the UK, the supermarket shelves are full and there is plenty of toilet paper. Brits can still travel all over the EU without needing visas, the only difference now being a visa is needed to work there. The same with EU nationals coming to the UK.
correct, they want qualified teachers with the necessary teaching qualifications. In short, they hire career teachers so if you want to teach in one then you will need to do the necessary qualifications and get some actual teaching experience.
as an example, Emirates were fine with my online Thai vaccination records (in English) when checking in at Heathrow for flight to BKK last month. No one asked for them at BKK, just passport and boarding pass.
they just want to see evidence you've been vaccinated. Whatever they use in China should be fine provided it's also in English. If in doubt give your airline a call as they'll be the ones who will decide if you get on the plane or not. I wouldn't worry about being checked at BKK, I'm not aware of anyone being asked to show it.