To quote the Simpsons: “0% is a percentage as well!” And that will be more than enough for politicians who know nothing about the topic and are blinded by the hot tech-buzzword of the minute (especially if it turns out they or some of their friends can make a shitload of money with it. I love capitalism and democracy.)
Wow, never heard of anything like it though it feels plausible that your ISP would be able to do this. I’m not an expert on this in any way though, just a self-taught, self-hoster. My first thought was getting yourself another router as well, though I’m not 100% sure if this would work. I dimly remember that I had to change my ISP’s router to “modem-mode” or something when I did this years back. And if your ISP doesn’t allows this …
Be aware that “everything” is probably untrue, though. Most of the custom ROMs still have a lot of Google stuff that is hard to replace / get rid of even with MicroG etc. (e. g. Captive Portal Check still phones home to Google servers on Calyx OS - if not deactivated in settings). Graphene (only on Pixel Phones 👌) probably comes closest. For detailed analysis check: https://www.kuketz-blog.de/android-grapheneos-calyxos-und-co-unter-der-lupe-custom-roms-teil1/
(in German, use (Firefox?) translation if needed)