Middling hardware aside, I've heard lots of online reports on unfulfilled orders and generally bad customer service. Something to keep note of.
Personally, I would stick with the GrapheneOS Pixel. If it works fine for your usecase, then I don't think you should change it out. GrapheneOS is excellent and I believe has some of the best security features of all the ungoogled Android ROMs. Its developer is pretty paranoid and they haven't been very nice to other projects though, which some might say is against the open-source ethos.
If you do end up disliking GrapheneOS for how it works and/or the attitude of their developers, I would rather install iodeOS (or e/OS if you like their iOS-kike interface + the Murena services) on another device, not the Brax3 phone. Possibly the Pixel 9 you already have (ideal scenario), but Fairphone is another option (I believe the 6 is supported by both iodeOS and e/OS). You might also like Shiftphone (which is a competitor to Fairphone in terms of their goal of making repairable phones)