That there are no penalty's written in DTV regulations is perfectly normal. That is not necessary, since it is about entering or being in the country. It is up to IO if you can enter the country, the IO decides if and what to check. If the IO decides you try to enter the country against immigration and visa rules, he can refuse you entry, and when he thinks that is necessary take other legal proceedings. That has nothing to do with a specific visa. Same is applicable when you are stopped by police. They also have the right to check if you are in the country illegally and take actions. Chances that any of these situations will happen are not particulary high, but certainly not zero. Telling the officer that you were under the impression there are no rules because you never saw an extra warning about this after you applied for the visa will not help.
where do you see requirements are void as soon as you are approved? A lot of people say so, but nobody can show any proof of that. I completely agree there are no checks at the moment other than on moment of approval and moment of in country extension, and incidentally when you enter the country. But having no checks does not mean the requirements do not exist any longer.
Indeed, rules do not change once thi visa is applied, so for DTV participation on softpower activity or remote working and 500K requirements stay valid the whole period of 5 years. This means that when they change rules by excluding activities or changing fonancial requirements for your existing DTV nothing woll change. You srill need the 500K for the whole period, nothing more and nothing less, and even when the activity for which you applied might be included in the future, for you it still be valid, but that does not mean you don't need to have any activity or no financial requirements at all. Just as in your example of the 20k with different age, the original requirements that were valid at moment of approval stay valid during the whole period the visa or stays are valid.
Here you give a good example, requirements of old retirement visa issued when limit was still 20K do still have the requirement of 20 K, so the requirement did no disappear after approval. Why should DTV be different and have requirements void after approval.
Where do you find that requirements stop at the moment the visa is approved, or is that just your guess? Where do you find you don't need to go to classes or is the fact that it is not checked for you the reason to say the requirement does not exist anymore. Read the rules, ontving soft power activity, medical treatment, remote work or digital nomad and 500k are VISA requirements, not APPROVAL requirements. At approval the visa requirements are checked by bank statements, contracts for muay thai, treatment plans and so on. Therevis no rule that tells visa requirements stop as soon as you got approved, and that works in the same way for other Thai visa.