It’s not super clear to me what your specific question(s) are. If you’re just wondering whether you should switch to GrapheneOS, then I can at least give you my insight / opinion.
Personally, I would encourage anyone and everyone to at least try GrapheneOS if they have the possibility to do so. It’s easily the most secure OS available, with far superior privacy than stock Android. It’s also extremely user-friendly, so while you will no doubt feel unfamiliar with Android in general, don’t let that discourage you from using GrapheneOS. The only real features that cannot be made to work are NFC payments with Google Wallet and running certain apps which arbitrarily restrict running on non-Google-certified devices (non-exhaustive list at the bottom) using the anti-competitive Play Integrity API.
As for a secure messenger, what is most important is finding something which the people you communicate with will actually be willing to use. You can look at this topic below for options, but Signal is my recommendation.
Google’s store accepts returns within 15 days of purchase, so you can easily try it for 10 days and decide if you like it. The reason I recommend returning the Pixel at that point is because honestly you are much better off with iOS both privacy and security-wise, than with stock Android on a Pixel.
Feel free to ask more specific questions if you have any.