Say you have 3 nodes A and C are in range of B but not each other.  C is a static node and A and B are mobile nodes

if A sends a message B can no it needs to rebroadcast becomes he doesn't hear C broadcast it.

but how can B know he needs to rebroadcast a message from C so that A can hear it?

Borrowing from Meshtatstic. If a mobile node doesn't hear a message from a at least two nodes and at least one of them is a mobile node it should rebroadcast it.

This still has the issue of  what if A isn't in range of mobile node D that B heard rebroadcast the message. but Meshtastic has the same issue so I think we can call it good enough for now.
might add some 'micro routing' for mobile nodes that instead keep routing to the whole network just to a static node. kinda like your star networking only the root isn't a single node but the whole static routing network.