kinda fixed it, I think...
Checked what image I'm using, its linuxserver, and they're using the OS's kernel implementation. Also saw that in my logs.
I used this https://www.linuxserver.io/blog/routing-docker-host-and-container-traffic-through-wireguard and added these lines into my wg0.conf:
PostUp = iptables -t nat -A POSTROUTING -o wg+ -j MASQUERADE
PreDown = iptables -t nat -D POSTROUTING -o wg+ -j MASQUERADE
Then I regenerated my mullvad config with a different server.
Now I'm downloading the Arch Linux iso with 11Mbit/s
Honestly I've no idea what exactly solved the problem, iptables, networks and routing still feels likes witchcraft to me most of the time.
I learned the other day that thats the reason you cannot make a folder named CON on windows. Dates back to pre DOS era.