Stop recommending questionable open-source like Matrix.
Synapse and Element are fully open source, there is nothing questionable about it. Having a company backing your project as main developer does not mean it suddenly becomes closed source or said company owns the project now.
None of the issues you mentioned are a big deal or make Matrix inherently worse than XMPP. The biggest flaw you can pin on Matrix is its performance but they're working on it.
CoreELEC is community maintained and the N2+ still receives the latest builds, my last update was just last month.
However, CoreELEC can be installed on many devices (including some Android TV boxes) that have Amlogic chips. You can see a full list if you to to the download page on the CoreELEC page https://coreelec.org/.
Also, CoreELEC is not Android, it is Linux running only Kodi. If you need anything besides Kodi you might want to look at another solution or have multiple devices.
As another mouse lover, the DualSense controller also has gyro aiming and I can only recommend it.
I'm playing Horizon Forbidden West with it right now and it allows me to be almost as precise as with a mouse. The adaptive triggers and the detailed haptics are also pretty cool. Also a really good controller for Switch emulation.
Now you can drag & drop all connections from your Virtual sinks to you output device (as shown in the image I posted). You can even send it to multiple output devices at the same time.
When you are done hit Ctrl + S to save your patchbay and select Patchbay -> Activated. Now qpwgraph will load your connections every time it starts.
On my desktop I use 2 virtual audio devices that are linked to my real audio card with qpwgraph in order to split audio between VoIP applications and desktop/game audio.
I also normally run in Flatpak but I switched to native until everything settles in. Turns out the config files are compatible now so you can run either and they will pick up the others config with no issue. Just have to watch out with shader pre-compilation because if you have different Mesa versions the clients will pre-compile back and forth all the time.
I don't run Pi-hole but quickly peeking into the container (docker run -it --rm --entrypoint /bin/sh pihole/pihole:latest) the folder and files belong to root with the permissions being 755 for the folder and 644 for the files.
chmod 700 most likely killed Pi-hole because a service that is not running as root will be accessing those config files and you removed their read access.
Also, I'm with the guys above. Never chmod 777 anything, period. In 99.9% of cases there's a better way.
Synapse and Element are fully open source, there is nothing questionable about it. Having a company backing your project as main developer does not mean it suddenly becomes closed source or said company owns the project now.
None of the issues you mentioned are a big deal or make Matrix inherently worse than XMPP. The biggest flaw you can pin on Matrix is its performance but they're working on it.