You won't lose your warranty, the warranty is mostly about the hardware, and you can flash the stock os again if it should be needed to send the phone in.
I use GrapheneOS daily on my phone and haven't run into issues so far. It runs stable, fast and I did not experience any inconvenience.
Only the sms app sends my SMS twice and sometimes I receive messages days later, but that could be a problem of my phone provider. Can't say what is responsible because I rarely send SMS and do most if not all messaging via Signal.
It can be in very rare cases that an app isn't allowed on grapheneos, for example I can't install the eBay app, it's not available for Grapheneos in official play store. But I don't care and simply use the website.
Overall experience is great so far, no crashes, very stable and running fast. Battery time is good to on a Pixel 9. If I actively use the phone, I get about one and a half days. If I just let it stay idle and connected to network, i get around 3-4 days. And this is with Google services running, without those it would last probably even longer.
What I like most is that there aren't any annoying AI services running.
Basic things like phone calls (also via loudspeaker) work without problems.
For passwords I use Proton Pass, which can be used on all platforms like PC, iOS and Android.
As a calendar I use Proton Calendar, which syncs across devices also.