I got your point and sure, the base layer makes a difference. Nevertheless the base layer remains unvisible unless you carry some tools and start digging. We can only assess the structure of the base layer by looking at the overall appearance of the track, the shape of the flanks an the top layer.
The tag representing the complete structure is the tracktype. The more development work has been invested into a track, the higher is the rank. For more details see Confused with tracktype usage - #7 by Map_HeRo.
The surface tag describes the surface only without taking care what may be below. Anyhow a rough gravel surface on top of a compacted base would not be much different to ride with a bicycle than a rough gravel surface on a rough gravel base. In both cases a pain in the ass.
A fine gravel suface on top of a compacted base would in fact be much different to ride with a bicycle than a fine gravel surface on a fine gravel base. A track consisting of 20 cm deep fine gravel would be impossible to ride with a normal bicycle but why would anyone like to construct a track like that?
Fine gravel is more costly than the base course mixture required for a compacted track base so why would one create a track of nothing but the more expensive stuff offering a worse durability and surface stability? I have not seen such a track in my lifetime so I believe this question is theory only. I’d say this is the reason why we do not have a specific tag for such “nothing but fine gravel” tracks.