Absolutely the easiest phone OS installation I've ever done. Completely web-based and easy enough for anyone to use. Even easier than flashing LineageOS.
The big hurdle to GrapheneOS is can you do without Google Wallet and other apps that won't work. The flashing process isn't something to worry about IMO.
I play Lichess on my GrapheneOS Pixel6a, works well. Same with Signal, Firefox with several mobile browser extensions.
Bitwarden, NewPipe, Tailscale, Duolingo, Uber, Discord, Matrix Element, all the Proton mobile apps, Backblaze, etc etc.
Pretty much every app I try works flawlessly. On rare occasion I'll experience minor bugs, and twice I've had to use GOS's extra privilege mode to get an app to work.
Overall, Love GrapheneOS and I'll use them as long as they are around and making an awesome alternative to Google's garbage.
I had zero problems with either FF or Signal after the first weeks though I migrated over to Fennec and Molly as alternatives since then. Sorry, I can't help with lichess though I can say that my nonograms work fine. =P