Freetube

mpv with appropriate plug-ins.

Various invidious instances.

Similar to Infinity for Reddit in that it is very user friendly. Good job.

Ubuntu but this is based on my experience from 15 years ago.

Archlinux irc used to be pleasant and helpful, not sure about the forums.

People need to experience something before they can ascertain if it is a viable option. Install Ubuntu or Linux Mint on a secondary device, let her see how familiar it is to her.

Then you can explain some of the other advantages of using Linux.