Is there a way to have templates inherit from templates? Like AppVMs inheriting from templates, but the AppVM is a template? This would be so that I can install many apps in minimal templates in such a way that I could have a “media consumption” template based on fedora-minimal-39 while also having the sys-net/sys-usb/whatever tools in another template (for the purpose of reducing attack surface; I don’t want ffmpeg or something used against me in sys-usb or something), all while being able to update a single “main” template for whatever is shared between all of those templates and not having to update the same thing n times where n is the amount of “sub” templates I have. This would also reduce duplication of things which are exactly the same, as at the moment I would need to clone the template to modify it.
With the current implementation it is not possible to get this
inheritance between templates
Sad. Is this architecturally impossible (for security/other important reasons) or is it just impossible right now? I read up on the subject a little bit and it looks like Qubes uses block device level “templating”, so no overlayfs is possible, could this be circumvented somehow via (e.g.) passing a bunch of block devices to the guest and telling it to overlay them inside the guest somehow (like in rc.local?)
Also, are there any workarounds? I guess I could install some flatpaks in AppVMs?
If you want to reduce the time spent updating the templates, I recommend apt-cacher-ng although it only reduces the downloading time (see unman’s notes: notes/apt-cacher-ng.md at master · unman/notes · GitHub)
Personally I’m more concerned with putting excessive wear on my SSD from literally having several OSes wanting to update at the same time, so unfortunately apt-cacher-ng doesn’t really save me here. 
With a decent SSD “excessive wear” is unlikely to be an issue.
A Samsung 850, rated for 150TB, managed 9100TB of data writes before
failure, and that was tested some years back.
If wear really is an issue for your SSD then I think you have other
things to worry about.
Oh I didn’t know I could write literal petabytes to low-TBW SSDs. Good to know. Thanks!