"Not trusting modern software to be correct nor secure" is sufficient.
I do almost all my web browsing in disposable VMs with no access to interesting things like my password manager, email, SSH keys, etc. I also run JITless (disable Javascript JIT engine), because those are a common attack point on browsers.
If you compromise my browser from a random site, you get nothing of interest. Even if you pop the kernel. You still have to get through Xen to get to anything I consider of value.
Browser exploits are a thing, and reliably compromise systems. Apple just released a security update yesterday for "something in WebKit," and we see regular browser security updates.
The art of escaping browser sandboxes seems to exceed the art of building browser sandboxes. The Javascript JIT engine gains you a lot of attack surface, unfortunately (one of the reasons I run JITless with Javascript).
As for snaps, they're just containers - kernel separated. Unfortunately, I consider the value of that against actively malicious code to be "about zero" - local root/kernel exploits are fairly cheap. Containers (so snaps) are great for convenience - if you want to run code you trust without worrying about dependencies, this is fine. They're not fine if you want to isolate things you don't trust - such as a browser from "everything else."
Qubes gives you a much harder boundary around your VMs than containers and sandboxes do.