I like this! (Even if it makes grinding worlds for them darned Water-Walking Boots harder.
) One thing I noticed though is that you didn't mention what happens when
it rains in the Volcano-biome. Maybe instead of rain, it could have some embers fall or a thunderstorm since volcanic areas are very prone to those? Maybe the Magma Golem could only spawn during a thunderstorm/ember rain like Ice Golem does, too. I always wanted a Summoner-Mage Hybrid Armor to balance out Frost Armor, usually being a dedicated Mage myself.
Edit: Oh! Oh! What if instead of Water-Walking Boots, Volcanic Chests have a chance to have Lava Charm to compensate?