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.

Go to android settings > apps > special app permissions (the last option) > alarms and reminders

The app causing the issue should be there, among others

Some apps that do some kind of scheduling display the alarm clock icon.

Do you have any app set to do something at a given time? Maybe an automated backup or some kind of automation?

::1

:D

Even the version number looks like an IP address

Theres even QoS in RFC2549

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.