You can easily switch back to stock Android if necessary :)
I switched a couple of years ago and the process then was pretty straightforward to the point I can’t really recall much about it, I can’t imagine its got trickier since then. I’m due a new Pixel sometime this year and I plan on putting Graphene straight on to it.
Process is simple;
Backup everything you want to keep and move the backup off your device.
Identify FOSS equivalents for all the apps you currently have (but maybe you already use them)
Read the installation instructions. Re-read until you understand exactly what every step entails and means. Any step you’re at all unsure of, ask. Much better to ask questions before you start than be stuck needing an answer halfway through.
If it goes bad (which it won’t) or you don’t like Graphene you can, as I said, revert back to stock Android.