Hi,
I use Debian 10 for sys-net. It works great.
Here are the steps that works for me.
1 - Install the minimal template for debian 10
2 - Update the template
3 - Install the qubes-core-agent-passwordless-root package (if you want)
4 - Shutdown the template
5 - Create a clone of the template (deb-10-sys)
6 - Start deb-10-sys
7 - Install the following package: qubes-core-agent-networking qubes-core-agent-network-manager pciutils
8 - Shutdown deb-10-sys
9 - Change the template for sys-net to deb-10-sys (after shutting down all qubes) and start sys-net
10 - If everything works (wired and wireless) you are good to go if not
11 - Start xterm in sys-net
12 - Use lspci to see the info of the card that is not working
13 - Search the web to see which firmware needs to be installed in debian to support that card
14 - Start deb-10-sys, install the firmware and shutdown deb-10-sys
15 - Restart sys-net - everything should work
I know I have use debian for firewall and USB in the past but went back to Fedora for USB because of some issue (don't even remember what issue) and I am now trying Mirage for firewall.
Hope it helps!
D.