Wouldn't it make more sense for Holmium Ore Melting to use a foundry instead of a chem plant? Normally I'd associate anything molten with the foundry. This would require going to vulcanus but I think that would be fine since players could still use the normal holmium solution recipe until they get the foundry.

Yes, melting it in a Foundry does make more sense.
Which is why you unlock the ability for using the Foundry as part of the third tier of research - Advanced Holmium Concentration and Crystallization.

My intent was having an option that you could get to initially even if you rush to Fulgora, and then advanced options that improve efficiency and productivity once your technology advances.