As I'm starting to run out of equipment space, I started looking more into how equipment upgrading works and figured I'd share some of the stuff I found:
Upgrading equipment is very similar to upgrading characters, cheap at first but quickly becomes more expensive. Equipment upgrades also seem to benefit from the same-type bonus exp (giving 1.5x the exp), with the type being either Ring, Earring, Bracelet, or Necklace.
Higher rarity equipment also seems to take more exp to level. The + value doesn't seem to matter for exp but the level of the equipment does. You can actually sometimes get more exp by leveling together 2 old equips before feeding them:
So to save space, I'm feeding 2 low+ equips that I'm not using together (preferably same type for the bonus). When I get 10 of these I'm feeding them as a batch to a high+ equipment I have that shares the type. If anyone has anything else to add or if I've said something incorrect, let me know!