Compartmentalization is only a part of the solution. Once you have that finished, you still need to deal with the actual vulnerabilities in guests, which will contain your secrets and be exposed to the internet, one way or another.
[0] https://www.qubes-os.org/doc/how-to-organize-your-qubes/
When I clicked the link I was expecting something like a unikernel, eg https://roscidus.com/blog/blog/2016/01/01/a-unikernel-firewa...
[0] https://forum.qubes-os.org/t/devuan-or-other-non-systemd-tem...
https://forum.qubes-os.org/t/alpine-linux-template-non-offic...
[1] https://forum.qubes-os.org/t/openbsd-as-a-ubes-os-component/...
[2] https://forum.qubes-os.org/t/mirage-firewall-0-9-0-released/...
Does it then become not a full OS anymore? Mirage is what I linked to above.
Probably not. I mentioned it, because you mentioned systemd. And yes, I saw your Mirage link and showed how you can use it on Qubes.