Yep, you can get any cheap or seemingly random domain and just have the zone set to your internet device(s).

There’s nothing stopping you, pushing your own internal CA, though, if you’re big enough to warrant that

I know, I run two domains at home on a Raspberry Pi, I know that it's easy, but I also don't have an issue with paying 10€/year for a domain name. I guess this is the thing people are angry about, that by buying a domain you're funding the very corporate greed that will one day destroy the internet, or something...