This is from the official GrapheneOS FAQ regarding different Pixel generations:
8th/9th generation Pixels provide a minimum guarantee of 7 years of support from launch instead of the previous 5 year minimum guarantee. 8th/9th generation Pixels also bring support for the incredibly powerful hardware memory tagging security feature as part of moving to new ARMv9 CPU cores. GrapheneOS uses hardware memory tagging by default to protect the base OS and known compatible user installed apps against exploitation, with the option to use it for all apps and opt-out on a case-by-case basis for the few incompatible with it.
Both 7th and 6th generation Pixels have a minimum guarantee of 5 years from launch. 7th generation Pixels are a year newer so they have an extra year of their guarantee remaining.
The Pixel 7 and Pixel 7 Pro are all around improvements over the Pixel 6 and Pixel 6 Pro with a significantly better GPU and cellular radio along with an incremental CPU upgrade. The 7th generation Pixels are far more similar to the previous generation than any prior Pixels.
Pixel 7a is nearly the same as the Pixel 7 other than having slow wireless charging and weaker dust/water protection. Pixel 7a offers the best value when buying a new device.
The Pixel Tablet is a tablet variant of the 7th generation devices and the Pixel Fold is a hybrid phone/tablet. These share the same SoC and are nearly the same as the other 7th generation devices under the hood.
Pixel 6a can still be worth buying if you can get a good deal due to the long support time. It still has just under 3 years of the minimum support guarantee remaining.
It is generally recommended to get an 8/9th gen Pixel if you are looking for the most security (hardware) features.