I don't know about soft power but I applied in Hong Kong for DTV digital nomad and other than the regular documents like proof of location, proof of being a digital nomad and proof of 500k THB, they asked me for 6 detailed monthly statements. The overall process was quite fast.
no fee for the 90 days report. Regarding the taxes that's a complex topic, but in any case if you stay more than 180 days in Thailand you become a tax resident.
I don't have a lot more info, but the 90 days report is something that pretty much all foreigners have to do regardless of their visa type (except smart visa) if they stay in the country for more than 90 straight days. If you search in the group you'll find some official documents (in Thai) saying DTV has to do 90 days as well... the information about this is not super clear so better be on the safe side and if you are getting close to your 90 days just go to the immigration office
no, my DTV was approved 3 months ago, this is the 90 days report that all foreigners have to do (except people on a smart visa) if we spend 90 consecutive days in Thailand
yes, if you hop in and out without ever staying for more than 90 days you don't have to do it 馃憤馃徑 I'm in Thailand for 3 years now (non-b + work permit before and now DTV) and this is the first time I ever did it