Flashing the stock Pixel ROM back is just as simple as flashing GrapheneOS, the instructions in GOS website are very good for both.

The only two things I can think of that might be issues are banking apps and Google Pay, if you use that. I use Play services in the main profile and honestly there's not much difference to the stock ROM in terms of user experience. Even Android Auto works nowadays.

For the banking apps, you can have a look at https://privsec.dev/posts/android/banking-applications-compatibility-with-grapheneos/. Just note that if your bank is not on the list, it doesn't necessarily mean it wont work.

I made the switch when I got a new phone. So I kept both the old phone with android and the new phone with GrapheneOS. There was a transition period when I would bring both phones with me, just in case. Now my old phone is my "whatsapp" phone which I keep at home and turn on rarely. During the transition period I used my old phone number whenever I needed to provide my phone to use a service, but eventually I transitioned that to a VoIP. But, even then, many services will reject VoIP phone numbers, so I still make use of the old one.

I had to request a special scanner from my bank because the banking apps do not work with GrapheneOS. And I had to make sure that nothing important goes into my gmail anymore because google would request that I used my old phone 2FA in the most inconvenient moments, and also I don't want to access google from my GrapheneOS phone.

I think that there are many annoyances that can and probably will happen if you try to jump right into GrapheneOS after having previously relied in the google/meta ecosystem. If you attempt to switch too quickly you might inadvertently lose access to your bank, and you might become suddenly unable to communicate with family and friends. My government's online identification system requires that I use their app, which runs on google services, so I still have to use my old phone for that. And I have encountered situations in which the only reasonably convenient way to proceed is to download an app. For example, recently I registered for a gym that would then require me to use their google-store app so that I could identify myself when purchasing a physical card.

I'd start by installing the apps you're familiar with (even non Foss ones) then moving to Foss app by app. its a lot easier to get used to one thing at once. that being said, graphene and pixel os are both android, so they're really similar. and the web installer makes it near impossible to mess up

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;

1. Backup everything you want to keep and move the backup off your device.
2. Identify FOSS equivalents for all the apps you currently have (but maybe you already use them)
3. 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.
4. If it goes bad (which it won't) or you don't like Graphene you can, as I said, revert back to stock Android.

GrapheneOS and Android are both based on AOSP, and calling/texting is such basic functionality for a mobile OS that I doubt that GrapheneOS' implementation is much different from Android's, if at all.

I've been on GrapheneOS for the past year or so, I've never actually enjoyed using my phone as much as I do now. No Samsung apps I can't uninstall, no Google spyware, the only stuff running on my device is stuff I want it to run, the way I want it to run.

Just do it. The web install makes it really hard to fuck up. And you can always reflash android if you want.

I don't see much at all that could go wrong with switching over to grapheneos immediately. I, admittedly, did it as soon as I recieved my device so there was no concern, but it was about as smooth as the process could possible have been. Back up any files you care about and just do it, I reckon

Everything should work perfectly fine.

Just make sure you know which apps are not supported on GOS such as Netflix, Hulu, Cash App, etc because of the Play Integrity API. Don't be afraid to use the Play Store as well. Its treated like any other app on the system so it isn't highly privileged.

Also, one thing that was a problem for me at first was the restore solution (and backup solution). You will have to transfer your files from an external drive whether its the cloud or a local one.

Feel free to join the GrapheneOS Discord/Matrix/Telegram server and ask more questions because they're very knowledgeable people that can support you.

Graphene has nothing to do with your sim or phone number, your sim card is possibly tied to your IMEI but that never changes so it should be just fine. If for some reason it won't work after you flash graphene your phone provider should be able to swap you to a new sim card and port your number over no problem, but even that would surprise me, it should "just work."

Btw Whatsapp and Gpay can run on graphene but it sort of defeats the entire purpose. Some people will create an alternate user profile for all that type of shit and install google play services on it, and put all their spyware apps over there. Just something to think about, it's easy to set up once you have graphene installed.

Well, you can always go back to stock by just downloading and flashing the image from Google. So that should be of some comfort.

You could buy a pixel 3 or even lower with the lowest storage possible to run lineageos to test if it suits your needs