You could try with attenuating the light source a bit, I use tracing paper on a custom box
The effect is quite notable:
(on the left is direct light attenuated at source, on the right is with the tracing paper screen between the light and the glass)
Unfortunately I don't own any coins like yours to compare; all my proofs are modern bullion which combine frosty and mirror devices. However you look at it, they are a nightmare to photograph. This morning I've been trying with an australian dollar, which is polished like stupid, and is just impossible to make it look attractive! Although it's an ugly coin, let's face it. Like 99% of bullion pieces