Neat game, got through the first iteration thanks to patient buffing and a lot of spikes.
The packing gimmick is very good, buffing items and armor actually made me reconsider the entire pack and try to put everything optimally.
Minor complaints I have is that all instants/consumables *must* be used in battle, it's just annoying when dealing with healing. That "summon ethereal sword" staff is terrible, cost me 30 health against olms, and I couldn't even fit it in the pack.
Played through the first iterationonly, and I don't know whether there's any reliable way to force builds, or just get what you want. Vast majority of items were discarded, and for a dungeon-crawler-themed game that's kinda sad, throwing all the loot away. Perhaps you could feed discarded items to the backpack and then "buy" any item you want once it's full?
The main gripe I have is action economy. Aforementioned staff costs 3 energy, all to create a sword that costs 2 energy to use, and it isn't even that good. Skull wand was my "get out of trouble free" trump card in the magma zone, given enough mana you can just delete all your problems. The deal with 0 energy effects is that they're infinitely more effective than anything else, even with "once per turn" caveat. Which is why shank is the most valueable thing to buff, it's always useful and more efficient than any non-zero-cost effect.
Complaining is easy, but the bottom line is that the game is fun.