If your ipv6 internet is broken you should probably turn it off on your router - hosts on the LAN can still communicate using ipv6 link-local, as some apps will want to do.

It is harder to maintain two networks instead of one. Potential problems double. Hacks like RFC8305 "Happy Eyeballs" become a must.

Fair enough. I do question it often.

It's a standard Asus router but it's given me a lot of ire. I hate to say it but it's never a problem when I install windows on the same machines

(I'm currently in the process of trying to completely remove windows from my life)