How can a digital nomad prove residency for a DTV Soft Power application in Thailand?

Citizenship should be enough, it is a higher state than permanent residency.With your passport you can prove citizenship.London embassy already confirmed citizenship and proof of location in the jurisdiction is enough. A hotel booking can be proof of location, but there are more ways to prove you are in the country.

Same boat as me. Digital nomad, dual citizen of two countries but resident of nowhere

As a citizen of a British or nation that has the king as head of state (Canada/Aus/NZ/UK etc) that country MUST accept your citizenship as proof of residency - this is your sovereign right as a citizen.

Therefore, if you apply from the UK or another of the previously mentioned countries, then an arrival stamp in your passport or anything that proves your location is enough.

This is the advice I was given by the Thai embassy in both of the countries where I hold citizenship

Ring a trusted friend in the uk and ask to be an additional account holder on their water bill - Thames water will do this with the main account holders authorisation.

Make a small payment of £5 on the bill from your name then generate a new statement or wait for the next bill. All future correspondence will have you listed as an additional account holder which is accepted as proof of residence (utility bill)

ED is a much better visa, but it demands that you study.

Wouldn’t change until after the ED runs out.

Firstly you would need to cancel your ED visa. This will probably require a formal letter from you to the educational facility to terminate your course and submitting documents to immigration after which your legal stay in Thailand will come to an end.

You MUST apply for the DTV from your country of nationality, you appear to be a dual citizen so chose whichever is better for your circumstances. You obviously need a criminal record check from that country and proof of residence which I appreciate is a challenge but you have to deal with that how you see fit. You may also be required to attend an interview at the embassy so applying from anywhere is not possible.

You can however attend any of the soft power courses whilst on your ED visa without needing a DTV although switching visa would seem to some that your intention is to live in Thailand with the least requirement to attend a course. I'm not saying that's your intention but it could be construed as the impression.

Ultimately you need to be in your country of nationality/permanent residence.

Drivers license with a US address is what I used. It's not as complicated as youre thinking

My son used scans of the front and rear of his UK Driving Licence to prove UK residency in his DTV application last week.

It was accepted and his DTV has since been approved

Don't you pay taxes ?

You should be able to travel back to the US and apply while in the US

You can not - so simple