Seems to me (and that doesn't mean much) to be the most secure / well updated option. GrapheneOS on a Pixel runs GREAT for me, I honestly expected a buggy experience from a custom ROM.

Also, my grandmother could install it (this was especially enticing, I was worried about having to flash an OS, didn't want to brick a phone).

You get a lot of flexibility when it comes to installing Google Play Services. What I do is install any app that needs gplay services to a separate user profile with them installed.

I honestly don't have any cons, I'm completely satisfied with it, I can't see myself switching back to an iOS device or trying stock Android.

I can think of two cons, although they don't bother me (not sure if this applies to every ROM available):

You miss out on some features / apps that come with stock android, such as AI features.

AOSP apps are installed out of the box, but aren't wonderful for day-to-day use IMO. I recommend Fossify and You Apps