if you're heavy using apple infrastructure (icloud, imessage etc) you're pretty trapped :)
for instance, if you have a lot of photos on icloud, you'll need to migrate them to another cloud storage, probably like proton drive or filen.
(and yes, obviously, you cannot install GOS on your iOS device, you'd need a pixel for it. refer https://grapheneos.org/install/web for more info regarding installation process.)