You dont need a visa directly for driving licence as such. You need proof of residency which is obtained from immigration. Immigration will issue these if your on a long term visa or extension of 1, a DTV should qualify as its greater than 90 days and you have a permanent address.
i suspect he made the mistake of going to the VOA desk first, rather than going directly to passport control desk. A lot of first timers entitled to exempt entry do that, myself included first time. Usually they get sent directly to passport control as their country isnt on the VOA list. Unfortunatly for your friend, his is, and the VOA team didn't enlighten him that he didnt really need it.
agree extensions have traditionally been 1900 regardless of length, but in all the official announcements describing the DTV prior to those issued to the consulates, etc (with no price shown), they stated it was expendable for another 180 days for another 10k. But your right people will bounce twice if they want 365 days a year.
i believe you were partly right, and in line with the intention of the visa. Officially if you want to stay a second 180 days , then an extension does cost another 10k. To do the full 365 days, you have to leave and come back utilizing the multi-entry even after an extension, which makes it a bit obsolete. So most people will bounce twice a year and avoid the 10k extension.
the no tax mentioned, is if you only stay for the 180 days in a tax year. If you stay 181 or over, you will be liable for any monies remitted into the country for the whole year.
going by all the other types of visa financial proof requirements, the 500k has to be a physical cash amount in a recognised institution. Eg BOI wont accept crypto or share portfolios as proof of income, and unless pension/income documentation is certified by an embassy this is not allowed. (Most embassies will not certify now). Bank statements are the normal acceptable documents.
At CW they do require a new copy of your marriage certificate each year (not the pretty part) that you and your wife will have to get from the Amphur before applying for your extension.