Really interesting game, I actually quite love the concept and I can see the potential in this. However I felt the execution could use a little polishing.
Firstly, the inventory seems a little clunky for a fast pace game like this. I actually like the tetris system, it adds more complexity to the game. However the problem comes from visual clarity, when you hover an item over the inventory, both it and your hand completely blocks everything behind it. I suggest adding some transparency for the item you're holding and your hand when you're about to drop it in.
However, I would say the most frustrating issue is when some items get launch behind the characters/mosnters, behind/below the inventory or even off screen. You could have them pickable even if it's behind something and have an outline or a silhouette of the item behind. Furthermore, give a little more warning time for when the item is about to despawn.
Finally, I have to talk about equipping an item, I'm not sure is this your intension but there it's quite easy to miss equipping an item when the character move and attack. Perhaps having a stagnant slot system to drop the item in might be better?
In anyways, I still enjoy the game and wish you all the best if you're planning to take it further. Thanks.