You know, you aren't the first person to mention GrapheneOS. I don't know much about it, but I'm half tempted to try it out.
Well, it is designed for privacy in mind. Doing so, it will void your warranty, so I suggest to buy used Pixel, such as Pixel 4A 5G for dirt cheap (for trying it out).
Secondly, since it has no Google services, you will find lots of apps that don’t work. Lots of apps, especially banking apps requires Google Play frameworks to work, so your mileage will vary.
You can install minimum required for apps to function, but I don’t know how much privacy will be comprised, if at all.