holocene here you can see the recommended devices: https://grapheneos.org/faq#recommended-devices
It's of course depending on your needs. With your budget I'd either look for a good deal on the Pixel 8 (because of the much longer support time and hardware security advantages), wait for a Pixel 8a (same reasons) or get a Pixel 6a or 7a now (whatever is cheapest per year left of support).
Any Pixel will give you huge benefits over other devices, mainly hardware security wise and the many features of GOS. If for whatever reason the choice is really only between the Pixel 6 Pro or 7, I'd take the 7 because I do prefer a non curved screen and a chip with fewer heat problems, also I have no need for the third camera and it's overall cheaper if you consider the support time.
Let us know what you ended up choosing.