Something I forgot to mention is that GrapheneOS still provides updates for older unsupported phones going all the way back to the pixel 4 with their legacy extended support. They explicitly state that it is only for harm reduction while you seek a new device, but it’s not like you won’t be getting any software updates. As I understand it’s really just firmware updates that you will be missing out on because those have to come from Google. It may be worth asking about your specific situation over at the GrapheneOS forum to see if they have any opinions/advice on whether legacy extended support would be preferable over some non-pixel with stock android from a security standpoint, since that seems to be the decision point for you: https://discuss.grapheneos.org/
Personally, I decided I will probably stick with my pixel once it gets to that time.