I figured out a solution to this "problem" a long time ago (before LAN sync even existed).

I have two accounts, SimpleX-Linux and SimpleX-GrapheneOS. For each contact, I create a chat where I add my two device accounts, and the person I'm chatting with does the same.

I name the chats in this format: "Russell<->TheFuzzStone", and I don't need to sync via LAN.

Simple and it works. :)