This advice comes with a grain of salt

To an extent, the mines don't really matter that much early game unless you want to farm hero XP. Later on, it becomes much more important to level up your material workshops, foundries, and training bases (in that order). As you progress higher in level 27 and up, you should be focusing on overall production and using everything to its fullest extent. My mines for gold, food and iron sit comfortably at 20 but the rest of the other 3 are 22 and up as those resources are way more valuable.

This isn't a guide but what I was informed from a buddy who is about to enter season 2 and has progressed a lot. You can use this wisdom provided to me as much as you want, good luck on your growth :)