I think this could be because of the way you implemented your block materials, as according to the wiki you should acually make a copy of a vanilla material type using FabricMaterialBuilder, as it then allows the API to impliment some mining level stuff for non-pickaxe tool types.
After some testing with Vanilla Excavators and this mod, I found that for some reason you cannot use the excavators to dig any gravel ore.
I read up on 1.16 mining levels for the API here: https://fabricmc.net/wiki/tutorial:mining_levels
I think this could be because of the way you implemented your block materials, as according to the wiki you should acually make a copy of a vanilla material type using
FabricMaterialBuilder, as it then allows the API to impliment some mining level stuff for non-pickaxe tool types.