I use CalyxOS on my Fairphone since 2022. It is better than the stock OS and allows re-locking the bootloader. It also provides timelier updates than Fairphone OS. It is absolutely fine and has zero issues.
I have one banking app that doesn’t work. Another one that does work. Also, I have paid purchases through the Google Play Store that do not see the subscription. I was going to let them expire anyway. I also have a Google One that doesn’t see the subscription, so none of the advanced editing features in Photos works. I assume all of these would be problematic on GrapheneOS as well.
I would run GrapheneOS if I had a Pixel, or if it supported the Fairphone.