I didn't switch directly from an iPhone to Graphene as I had another android phone in between the iPhone and my Pixel, so that's my point of comparison. But I found the switch incredibly easy. I got used to the UI within an hour or two, transferred important information, and never looked back. And once I switched to Graphene I had no major issues or difficulty adjusting when comparing to the vendor OS on either android phone I've owned. It would have been almost as easy switching directly to Graphene, with slightly more of a learning curve given it's more bare-bones.
The only part I'd say was even slightly difficult/frustrating about the switch was replacing Apple/Google services around the same time, as I had to learn how to administer a server, self-host applications, run reverse proxies, switch to a different paid email service, etc. But it was nothing too bad and I figured it out fairly quickly. Sounds like you're already doing that, so you should be fine.