What are the requirements for extending a DTV (Digital Tourist Visa) in Thailand, especially regarding bank balance?

Wise move with the boarder run I’d say, seems to be a lot of scrutiny with the 90 day extensions

When you apply for an extension at immigration office, they just check if you still use the visa correct and comply to all requirements. You should have the 500K always when you are in Thailand.

Only when you still comply, you will get the extension.

At the border they do not check most times.

When you book a border run, they wil also arrange a smooth operation for you at immigration, that is where you pay for.

From what I've been reading here it's much easier and less hassle to boarder run

In country extend = same documents as you applied DTV

500k equivalent for the last 30 days and a complete update of ALL other paperwork would be normal for extension

Is this for the 90 day report?

Yes, these are the usual requirements when extending your DTV with immigration.