This mod is good. However there are a couple of tweaks you can do to make it better. Here are all of them:
1. Change sprite of upgrade matter not to be exactly the same as bow matter's one. Having same sprites can confuse people.
2. Slime essences are dropped from every blue slime and only used for one pretty weak bow, and even doesn't worth anything. They are pretty useless. Can you, for example, buff that bow and make essences drop from king slime? Also, they can be a material for certain potions.
3. Balace your mod. Skull crossbow is too OP for a weapon made from book of skulls and one upgrade matter, mechanical bow (by the way, why is it named "bow", if it is a crossbow?) lights up the area crazily, enchanted bow has more DPC than enchanted sword. You understand, what I mean. It is NOT fun to play with Overpowered mod like this mod is now.
4. Improve older content. A lot of old bows are just regular bows, which shoot normal arrows. This is not very cool considering you can upgrade them and make better. Destroyer's bow for example can shoot probe, which will shoot lasers at enemies (like magnet sphere), and Cthulhu's bow can shoot arrows, which on contact with ground or enemies will split into small homing eyes, slime bow can shoot sticky arrows which will stick to enemies and deal damage to them (like bone javelins, made from desert fossils), strange bow can shoot one of four different types of custom arrows (because there are four types of strange plants) which can have some unique effects. Also, pink gel bow shooting chlorophite arrow looks strange, so can you make pink gel arrow with same AI to fix that (also it's underpowered as some other bows)?
5. Party zombies are TOO powerful for eart
ly prehardmode.
6. Lastly, I will suggest you to resprite A BUNCH of stuff from your mod. Considering that Blossom bow and star bow look good, I think, that you already got spriters, which will do that, and you probably realize yourself, that sprites are the biggest problem of your mod, so I put this suggestion on the last place.