My experience so far as a new user, which might be a little redundant but here goes:

• Overall, there is a balance to work out between security, decentralization and FOSS, and anonymity.
• for the average user, using sandboxed google play is pretty much essential. Otherwise you'll spend days trying to figure out why you aren't getting notifications, why certain integrations aren't working, etc. Notifications especially are just painful without google FCM. HOWEVER, I do not believe it is mandatory to sign in to your Google account for notifications to work, so you could in theory avoid signing in at all and still take advantage of FCM.
• multiple profiles don't make sense for my use case (and possibly most people). Graphene does advertise the use case of having banking apps on a separate profile, but after attempting to do just that I believe it is a very niche use case that would actually benefit from it. Obviously a great tool to have for privacy and security, but not something you'll went to use everyday.
• For the move away from Imessage, it is indeed kinda painful and still ongoing. The simple fact is that people are super weird about switching from I message, and honestly going straight to Signal was a no-go for many of my contacts. I've had to settle for WhatsApp, Telegram, and even Discord... I just have had to accept that the transition will take time. I've weighed that privacy issue against the privacy gain of GrapheneOS itself, and the benefits of supporting a 3rd party OS option, and I still believe using Graphene is better overall. And, once people get used to using a 3rd party app vs Imessage, in a couple years the jump to Signal will be no problem at all.
• banking apps are super painful. That being said, here is an opportunity to vote with your wallet... Support apps that don't require invasive system access for "security". For me, the biggest eye opener was that there are NO GENERIC THIRD PARTY TAP TO PAY PROVIDERS IN THE US. It is only Apple Pay, Samsung Pay, or Google Wallet. And, as is pointed out on the Graphene user guide, 3rd party apps are allowed to implement their own NFC payment system, but the extremely vast majority simply choose to use Wallet or Apple Pay. This is obviously rather scary as more and more retailers use these systems, and I've realized I would gladly support and use any alternative at this point. Without Graphene, I would have never even thought about it.