GrapheneOS only supports Pixels due to hardware security requirements as they can use the phone's secure element. Other manufacturers either don't have one, or they permanently disable it when installing another OS (Samsung).

Pixel phones are sorted by the main model, the Pro model, and the A-series model. All the phones in those generations use the same processor so there isn't much of a speed difference. A-series ones are the cheapest and have the lowest spec camera and display, while the normal and the Pro have better versions.

The A-series ones always release last of their generation so they always have longer updates (7a's updates end later than 7 and 7 Pro).

If you don't care about the better display or camera go for the latest A-series model. If you want a tablet then Pixel Tablet is best bet and if you really want a folding display then the Pixel Fold exists.