Belt bottlenecking problem

Slowly incorporating powershards to maximize my mk 2 miners and realized a problem, currently at mark 5 belt and it's capped at 780 and I have a miner outputting 900 coal, so obvious solution is a mark 6 belt but haven't unlocked it yet and I see mk 3 miners can be unlocked. I was also thinking of a splitter but I can't directly feed the miner to the splitter, is there any other way to maximize the output or am I really capped at 780 for now?