Do I need to return to Thailand to keep my DTV visa active, or can I enter anytime within the 5-year validity?

Anonieme deelnemer when you got out the country already after the moment you should report, better leave it for now. Since yhe counter resets every time you get in, at the moment you are OK. They can see you missed one report, but normally this will not harm you, only when you need something from immigration they might fine you for this.

The reporting is only every 90 days when you are in the country, counter resetting when you reenter.

So when you travel outside the country at least once in 3 months, you never have to do this reporting.

But since it is free and only passport and TM30 is needed, it is not worth to do a border bounce only for this.