I know everyone is mentioning Backpack Hero but I think your game is a cool twist on the concept.
Actually playing through the battles is an absolute slog, I'd recommend making every nugget of energy you find give you 3-4 energy instead of one or two.
I think the enemies should also have a random chance of failing to gain energy every turn, to replicate how the player isn't the only one trying to mine energy mid-battle.
I think you should change the batteries that let you keep energy between fights, and instead make them give you a set amount of energy at the start of a battle, the way they work right now encourages grinding to make sure you end battles with as much energy as possible.
You should give WAY more items at the end of battles, since the main limiter is your ship's equipment slots, you want the player to focus on making meaningful choices to pick out the best gear for their strategy. When I played through, I didn't have to think too hard about what items to leave behind because most of the items I was given were duplicates or didn't seem useful enough to bother with.
Don't give up! Making games is genuinely hard and I think you have a good idea here.