Hi there, I think you should be able to use repair kits on all levels of tools, otherwise there is no point in them even existing, as you could just carry the same material and repair it raw anyways.
Even if it took 100 simple repair kits to repair a steel tool completely, this would still be balanced as you would need an excess of the lower tier material to repair the higher tier item.
There is no point in requiring repair kits made from steel to repair steel tools, i will just carry the raw steel instead. This is currently an absurd, redundant, unneccessary system/item.
I should be able to repair star metal tools, at a reduced rate, using many repair kits made from steel. This increases the inter-connectivity of the material economy, and allows the smart use of any excess resources you may have at the time, despite w/e tier of tool/weapon/armor you are using.
The extra cost would be the time spent making the repair kit, the forethought and requirement of making them at a bench, and the reduced effectiveness using lower tier repair kits on higher tier weapons.