Check your DHCP Server and disable DHCPv6. The raspi shouldn’t get a ipv6 then and you won’t have this problem in the future with other devices.

edit: and restart the pi after disabling DHCPv6

You can also just block YouTube in your unifi firewall/router for that one client.

::1

:D

Even the version number looks like an IP adress

For your usecase a pi should be sufficient. You can go with a pi 5 8GB and a docker install, so you can host more stuff later. I would recommend an m.2 head with an SSD instead of an SD card though. Much faster and more reliable.