Cube
see https://www.gnu.org/distros/common-distros.html#Debian
Personally I think Debian is an excellent libre distro.
Tails shouldn't affect matrix messaging other than the session always being temporary/amnesic (which I would find tedious).
Given Tails doesn't come with a matrix messenger either use a matrix client in the browser (eg. element web) or install a matrix client (instructions for installing additional software on Tails).
I've not used persistent storage but perhaps it can be used for remembering matrix sessions (?).
If you are okay with other messengers, Tails includes the Pidgin messenger which can connect to XMPP and use OTR encryption. Tails also includes OnionShare which can also be used for messaging over Tor (I don't know whether the included version supports that).
This looks awesome
Youtube still has pretty much everything unfortunately - there's not yet a proper alternative for content (depending on what you want). I use Newpipe and Freetube to watch it (I would use RSS with mpv if I didn't have so many subscriptions).
PeerTube isn't a replacement but it's a fascinating alternative with some good channels/instances.
https://joinpeertube.org/
In addition to the other LBRY/Odysee comments, see this recent news: https://odysee.com/@lbry:3f/lbryandodyseeevolving:7 (Odysee might not/probably won't be open source in the future)
May Evil Track All
The BitTorrent sharing is done in the browser so if you have the option (My settings > Video settings > Help share videos being played) turned off or WebRTC turned off (including Tor Browser) or are using other methods to watch videos on peertube (eg. downloading) it shouldn't be torrenting anyway. Issues with BitTorrent with Tor also apply here if enabled.
Read: https://framatube.org/about/peertube#privacy
Also watching videos will probably be quite slow over Tor, so I wouldn't recommend it. Perhaps you could use a VPN instead, although I don't know your threat model.
The dystopia is already creeping in
I would be a waterbender
If we keep repeating this question I think that'd make us lemmings
I'm pretty sure these are hand-picked