Graphene OS drops support for devices pretty soon after Google does. It’s not a good idea to buy anything before Pixel 6 to run Graphene OS right now (see https://grapheneos.org/faq#supported-devices)
This will get better going forward since Google is supporting the Pixel phones for much longer (5 years for the Pixel 6/6a onward, 7 years for the newest devices).
The x390/x280 are the same era as these but smaller, so might be a better fit here. The X390 has soldered RAM though, so I’d look for the 16GB version if you can find it (there’s not much of a price difference used)