The main reason for splitting up the relays into two sets is that with 10A traces the connector pins would violate minimum separation distances. I would have to get even larger connectors. The ones in the design have 5 mm pitch.
I’m assuming the original board also did this for the same reason although their board is set up for 8 motors split into sets of 4.
I’ve also had some trouble sourcing a 10A fuse suited for inductive loads but I’m sure I could find one with some more time.
The whole board is on its own house circuit fused with a 16A breaker.