Do general tracking blockers and mainstream lists block GTM though? I use a massive combined listed on a pi-hole so not sure which one it comes from. Just curious as there are, theoretically, non-invasive, non-ad tracking use cases for GTM.

I think Google ran out of good will/benefit of the doubt when it comes to tracking tech a long time ago. AFAIK, every blocker tool I use on any platform, which probably includes most of the well-known ones by now, will block GTM in the default configuration.

If a site doesn't work properly with GTM blocked, legitimate use case or not, personally I normally just move on. Unless I have a compelling reason to need that site, I won't make any attempt to work around it or change my blocker settings.

The only Google property I'm aware of that isn't routinely blocked by these kinds of tools now is Google Fonts, and I think some of them even block that by default now.

GTM is usually the wedge that marketing uses to pry its way into a site's code without engineering involvement, and thus blocking GTM will often prevent a huge list of other trackers, ads, and late-loading content changes from being loaded.