you will want to reference Germany Thailand tax treaty (I assuming you are German). That treaty is a bit more weighted to Germany than others. If you are consider a tax residenct of Germany and you have permanent home there with closer economic ties, then it does give you a skategoat from being a Thai tax resident.
Most treaties are not so lienent though, and I'd you don't have a permanent home there, then you would be still need to pay tax in Thailand. See the treaty that applies to you for more information.
if you do formal learning, like at university, then you can get 1 year visa.... Most Thai schools do not offer formal learning, that is why you have to get regularly extention. Set that as your expectation going forward getting a visa for learning Thai. You can look at university programs studying Thai if you want a formal learning.
just for thought, its a well written review. The visa fee and extention costs are very normal, but you make it sound like a scam. Very 60 days extention for informal learning and immigration charges. It's also typical to charge for visa assistance, they have to prepare many documents and submit to MOE. That's a cost to the school not included in course work. It is sad that they took 60 days to get you your books though.
With Thai insurance, it's typically excluded for every. AXA international plan does come with some limited benefits for preexisting with waiting period. However, certain policies can be negotiated to be included for additional premiums, talking to an agent is probably good. You can probably find a few in this group, most expats recommend AA Insurance brokers, but best to check around too.
MMR, TDAP at the very least for the safety of you and others would be highly recommended. Hepatitis is also smart. As far as travel based recommendations, typhoid would be at the top of the list.