I'd say NixOS is great for servers, mostly. Only having to worry about certain things (secure boot with custom keys, FDE, partition layout, network, sshd, firejail, etc.) once, and then replicating the same setup on another machine is waaay more convenient than going "I wonder what I was thinking when setting up this machine" once in a while when looking at some machine again you haven't touched in some time.
When it comes to desktop usage, the whole thing does not feel as magical - configuring system options in e.g. KDE is still a lot of clicking around in a GUI. I still use it for my desktop machine, just so I don't have to think about another distro.
These containers are running on various servers I have at home, not on a desktop machine. I use podman as an alternative to docker, because it's fully libre and does not require running containers as root. To be honest, I've never thought about running flatpak containers for these kinds of services – do you have a setup like this that you want to share?
No, your VPN provider needs to offer port forwarding for that to work. You can still use qbittorrent without port forwarding, but it'll be harder to find peers which could lower your speeds
AFAIK the selling point of gluetun is that the traffic from e.g. a qbittorrent container is guaranteed to go through the VPN - this is also what one would need port forwarding for
I'm pretty sure it's pronounced mü, not mew