Again I disagree with your interpretation. For any embassy to accept learning Thai for DTV it must be within the official catchment criteria, unless you are suggesting the embassy's that do accept are wrong. As is broadly seen Thai Embassies seem to apply individual policies some of which are more current than others, just as some IO's apply more stringent examinations on entry than others.
' filling in a missing part in the Thai visa schemes' is an evolution of those schemes.
I disagree with your interpretation which to me appears not to reflect the evolution of the Thai visa schemes. The use of DTV instead of ED visa is just part of that evolution.
You infer learning Thai in Thailand is not a valid purpose of stay contrary to that specific criteria being allowed under the DTV and being accepted by various Thai Embassies across the world?
How is finding a criteria that fits your circumstances ' tricking the system' ?
The 'system' evolves and changes over time, as folk are now finding with extending the DTV even immigration offices are not sure what to do so an equivalent of the 'border bounce' is easier.
Getting the visa is likely to be the hardest part, as it's 'out-sourced' to a commercial company (like many traditional civil service functions) and with no 'appeal' possible there is a perverse incentive to deny, thereby generating another application fee. 😡