I’ve managed to stay on SailfishOS for a long time recently but in the past years I used to switch between SFOS and GrapheneOS (GOS) about every six months because something started to annoy me and then I switched and then something started to annoy me there and I’d switch back 
Anyway, interface-wise GrapheneOS is just stock Android which you can either run degoogled (more or less like running Android apps on SFOS) or with sandboxed Google Services in which case almost every app works like it would on “normal” Androids.
(Below the surface the appeal of GOS is that they’ve hardened it.)
So for your use-case, Firefox WhatsApp and mail, there’s not a big difference between either SFOS or GOS.