I would propose the next scenario for DoH repairs.

First of all DoH repair should use like 4x Dark Matter (with it's cost reduced 4x) to be able to repair a fraction of damage (for example if item has 80% durability it currently has no sense to do it) and in this case you will be spending 1 Dark Matter for every 25% repaired.

Next Repair should be a General action with interface similar to NPC repairs with a list of all damaged gear and repair all button. You may highlight gear which you can repair (having respectful DoH class leveled) and instead of total cost in gil you get a total amount of Dark Matter consumed.

You may later implement a feature like "Request Repairs" similar to "Request Meld".

Though I do understand that Melder NPCs are currently one of the best gil drains but you can actually increase repair and dark matter costs if you can implement simple and convinient DoH repair system so that DoH repairs will be slightly cheaper than current NPC and new NPC repairs will cost more.