gk7ncklxlts99w1 I recall the GOS forum account mentioning that GrapheneOS is more secure than any desktop OS right now (which I think is very likely).
That's not just very likely, that's a fact. Desktop OSes don't even come close.
gk7ncklxlts99w1 Would I be correct in deducing that I am less likely to be compromised by a malicious email attachment on GrapheneOS, than I am on desktop?
Generally yes, but this also depends on the programs you use and the settings of the email program, for example it is important to deactivate scripting and dynamic content.
gk7ncklxlts99w1 If I ever need to use my email on desktop, I would use it in Brave browser installed via flatpak
Using a browser for this is a good way to view emails and their attached PDFs, due to the browser's additional security measures like sandboxing. But using the browser as a Flatpak is not a good idea, since it weakens the browser's own sandboxing.