I'm a bit surprised you haven't had a response yet, I was curious to see the answers to your questions myself since I've only used GrapheneOS for a few weeks.
I think you're correct that google play services would need to be enabled to make app purchases. I too wish I could get away from that entirely but need it for a few apps.
I've been trying Magic Earth for navigation but unfortunately it just doesn't have nearly as much as Google Maps has. I've contributed hundreds of houses and addresses to OpenStreetMaps over the years and it's discouraging how many there are still left to do.
I've run into a few issues:
- Emergency alerts don't seem to work when my phone is in do not disturb mode. It's hard to test but my phone played no sound. So now I have to sleep with the phone in normal mode, for safety.
- NFC just doesn't seem to work. I can live with that though.
- Boarding passes on the lock screen do not work. Not a big deal but one I spent a lot of time trying to solve.
- Visual voice mail does not work (Mint Mobile). I get a notification saying I got a voice mail and then I have to call the number but I cannot listen to it directly through the phone app
- I tried using the stock SMS/MMS app but several of my contacts were on Apple and using RCS and I had no idea until we realized we weren't getting each other's messages. I would love for everyone to use Signal but I can't expect them to or to disable RCS for me so I had to install Google's Message app and all of the messages suddenly came through.
So I think I have Google Play Services specifically for Google Maps (when I need to do a search for something) and Messages.
Some cool alternatives I found for some other apps that you might find helpful:
- Launcher I went with Lawnchair which has a lot of the same features as Nova but without the company behind it.
- Podcasts I went with AntennaPod, which has a lot of the same features as PocketCasts
- Keyboard I went with Futo, which I know many dislike around here but I like swipe-typing and options are pretty limited in that space right now. It's a pretty solid keyboard.
- Stock camera is pretty limited, I installed Open Camera as a backup.
- Stock clock is the only one I have found that seems to actually function, the others have missed sounds, timers that just pause mid-way etc., even when battery optimization is turned off
- I liked Fossify Contacts better than the stock contacts (which froze in its search capability a few times)
- I needed to access SMB files so I use Material Files
For a few apps I had to turn on the "Exploit protection compatibility mode" in order for them to function. Typically banking apps.