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.