It has been made ABSOLUTELY clear to me, by my local Thai embassy (Manila), that for one to apply for a DTV, one is required to show evidence of regular income, over and above the half a million baht in one's bank account.
Obviously, if one has a regular income, one would probably use that fact to apply as a "digital nomad", rather than under the "cultural activity" or "medical" categories, so this visa seems to be a farce!