"[GNU/]Linux being secure is a common misconception in the security and privacy realm."
https://madaidans-insecurities.github.io/linux.html
"[GNU/]Linux is thought to be secure primarily because of its source model, popular usage in servers, small userbase and confusion about its security features. This article is intended to debunk these misunderstandings".
Based on this, one should try to do as much as possible on a GrapheneOS device
GrapheneOS is still not perfect. The general consensus among people is that running QubesOS with a Whonix/Kicksecure container is the best you can get atm but even that it is not perfect.
The point of the Linux insecurities article is to fight common misconception by the FOSS community that using a Linux distro is going to solve every single security concern you might have. It does not mean, however, that Linux is inherently insecure and shouldn't be used.
The author himself had said he uses Linux and Firefox despite what he wrote in the posts.
The real point is not that Linux is less secure than often said but that "inherently secure" is not a thing, especially not when a network is involved. Your system can make it easier for you but you still have to look after your own safety.
That's pretty much what I have said
Yes; I was summarizing, not offering a differing viewpoint.
I would say QubesOS is for sure the safest, but having normal sandboxes and permissions should be enough. QubesOS is like making an insecure OS secure, as there are no permissions or portals, so you need to go way beyond and run multiple VMs at a time. This is not suited for any daily use, my modern laptop really struggles to run 2 VMs at a time