Fairly new user, since December, on a 9a and have already stated elsewhere on this forum that I've never been happier with a smartphone than I am now. That statement still stands, and it's all due to GrapheneOS itself.
To answer your questions in rapid succession:
- Nope
- Nope
- Nope
- Nope
- Yup
- Android 16 does have a "themed icons" experimental feature for the default launcher. Not a developer option, or anything.
With that said, opinions differ with taste. I discovered μLauncher, through Accrescent, after the switch and quickly adopted it for my homescreen needs.
Run it without icons at all, just listings of apps + gestures for the everyday stuff. No worries about using a third party launcher at all. Particularly this minimalist one, with no permissions...
Similarly, I don't mind using Aves for my gallery needs.
Messaging is preferably done through Signal (Molly), so the stock app is not an issue for me. Though for whatever SMS needs I might have, I'd rather use the stock one due to security. Both SMS and MMS have been used as attack vectors before.
I say go for it!
Personally I can't stand iOS's environment, and was happy to get away from it after having to use an iPhone for work.
Like I said, opinions differ.