Do I need to leave Thailand every 6 months on a 5-year digital nomad visa?

Yes

It's a Destination Thailand Visa (DTV) and you can extend once in country after your 180 days stay but the paperwork to extend is not worth the effort, it's the same as re applying for the visa. Better to leave the country for a short period then re enter and get a new 180 day stamp.

You have to do a border run every 180days OR apply for a extension at immigration but can only do once per entry

There is no "Digital nomad visa"

If you have a DTV you will receive permission to stay in Thailand for 180 days every time you enter Thailand. You can apply one time per entry for a 180 day extension, but most immigration offices do not want to grant this extension and the required paperwork to apply is extensive. But you can just leave and return to get a new 180 day stamp instead.