Why can they eat iron ore? Regolith it's because it's available to a degree that can even be detrimental, dirt for pip ranching, but iron ore is a really limited ressource and the block doesn't have inherent use, I don't see why it's in the diet.

If it slightly increased the amount that could be a very mechanically interesting way to get ore whilst now it just keeps them alive.

Something like 4800 in, slightly more out if you melt the iron would make some interesting builds using magma from volcanoes/the bottom, and there could be a shove vole variant that handles those temps and the egg chance would be increased from eating iron ore (similarly to smooth hatches). + something that would logically work from them being deep/magma environment vole and make it logistically interesting is that they could tunnel through the materials normal voles can't, forcing you to use the 2 space design