I had thought about removing the metal bases but the figures themselves are cast in a fairly 'bendy' metal and I decided not to risk it.
Difficult - you have gone to a lot of trouble with your 'stone' effect bases and the metal base on the model does detract from a very unusual unit.
I have recently been basing a lot of non-slotta models and after gluing them to a 20mm base, or cavalry base in your case, I have just used Polyfilla to level up the base. Ok I was going for a muddy or field look, but that type of filler is easy to smooth out and you could probably use some sort of tool to scribe in your stones.