At least on a LAN, you can set up like fe80::3 . I think. Now I'm not sure if I got that right.

fe80:: is for link local. You'd want to use something starting fc00:: or fd00::

In your typical home environment, just set your ULA to fd00::12 instead of 192.168.0.12, or fd00:16:34 instead of 192.168.16.34

Yes you'll run into issues if you were to later want to merge your private IPs with someone else, and you should use fd12:3456:7890::12 instead, remembering those extra 10 digits, but its not a problem at home, and no more of a problem with business mergers than ipv4 clashes anyway.