qubes-mirage-firewall VS qubes-app-linux-usb-proxy

2) https://github.com/mirage/qubes-mirage-firewall is by far a better firewall for Qubes than OpenBSD ever will be - unikernels are far more secure than a traditional operating system is and you can read all about it on https://mirageos.org/

That's correct. It does mean that the closest to a self-contained program you can run is a unikernel like the mirage-firewall, unfortunately. On the upside, those remain easily portable to essentially anything that can run VMs so long as you adjust the image format.

That sounds similar to a unikernel. There are actual uses for those in seL4 and Qubes OS such as a firewall-qube (in theory unikernel qubes should be able to take far less system resources to run than full Linux+distro qubes).

Here's one that is "production" ready: the Mirage-Firewall microkernel running on Qubes OS.[0]

[0] : https://github.com/mirage/qubes-mirage-firewall

sys-net, sys-firewall and other administrative vms should slowly migrate to unikernels instead of running linux, which should help with ram usage. The mirage.io project seems to build a couple qubes vms, for example https://github.com/mirage/qubes-mirage-firewall is a firewall which they indicate to give 64Mb of ram.

Regarding USB, yeah, the usb-proxy doesn't have support for all things. I think it comes down to stuff not implemented in Linux USBIP or USB support (I've had issues with some device controllers not actually implementing what they claim to implement).