You won't see 18 months on an Ed visa anymore unless its an approved university course. Probably 6 months max. So your only current option is the DTV, but dont bank on the incoming government keeping the DTV alive.
As others have said you can legally work online on a DTV, but not on a retirement or education.
You cant have a Thai bank account on a dtv and immigration dont want anything to with you if you want to do extensions, cancel the visa etc.
Got asked similar more than 10 years on a business extension. Nothing new. Was under a Taksin government so Taksin had probably made visas to troll then as well causing a crackdown.