> I don't want anyone to know how many devices I have at home

Even if your ipv6 host or border firewall allows pings through, it's not practical to scan an entire /64. There's just too many addresses in it, and your devices will frequently change them.

> I don't want anyone to know which one I'm using to access their website, I don't want anyone to try guess the OS and version of my devices, etc.

They already do this through fingerprinting that operates with higher-layer protocols.

> And now I'm scared to have a simple DLNA media server because I can't just install WireGuard on the TV.

This is very simple to implement. Ensure it's listening on the link-local address. That's the IP that starts with fe80. These are unrouteable by spec.