VAULT And in all honesty (no offence intended to anyone) I feel like the 6a seems to suffer more issues, as when I see a "problem" post in the forum and click on it, it seems to me its more often a 6a than other models. I don't want to suggest the 6a if it's going to give them more challenges.
When installing GrapheneOS on a Pixel 6a, OEM unlocking won't work with the version of the stock OS from the factory. You need to update it to the June 2022 release or later via an over-the-air update. After you've updated it, you'll also need to factory reset the device to fix OEM unlocking. That's the only "negative" thing that I can think of when it comes to the Pixel 6a.
I've used GrapheneOS on a Pixel 4a, 5a, 6a and I'm currently daily driving it on a 7 Pro. The 4a and 5a were, by far, my favorite in terms of the device design and the 7 Pro in terms of hardware.