As Torinir said, you can just use the stars [...]
Ehh, that's what I said, too.
The stars are meant as a debug tool primarily, (or probably an expensive endgame tool, once the game's economy is implemented,) they're not meant to be spammed 100+ times to unlock everything in one sitting unless you're cheating, even with the game's current economy.
Capturing stuff for health is great too, but outside of the sheep and the snake, there's not that much incentive to capture monsters (yet). If the health drain is truly meant to be a permanent thing, there needs to be:
1. More indication she's actually damaging your max health and not just dinging your stamina bar. (Like that shadow-y, electric effect I suggested should go on your health bar.)
2. More indication it IS permanent and that you need to do something about it.
3. More incentives to give capturing a try AND stick with it. As of right now, you could 100% avoid capturing anything and still manage to play and beat the game just fine (outside of the snake and sheep, I mean.)
Or maybe if you feed her enough of your HP, she'll get "full" and she'll leave you alone and flat-out tell you what she's been doing (instead of all the vague, very trope-y succubus dialogue) and tell you how you can go recover some of it in the hopes she can "make some room" for your energy eventually and go back to getting good meals out of you.
Ideas for #3:
A. Food isn't bought from the shop anymore, you have to go get it yourself. Meat is earned by capturing monsters. (Carrots would probably be found during drilling, capture a beelike enemy to get honey, etc.)
MAYBE there's a hunger mechanic now too?
B. All/most of the vehicle upgrades are done via monster parts (kinda Monster Hunter vibes), collected by capturing them and doing good at the shoot-em-up minigame.
Maybe there's a durability/repair mechanic, to force players to have to do it again every now and then (not too much, pls), instead of unlocking everything one by one and being done with it, IDK.
C. Every monster type has a rare one-time drop they have a low-ish chance of dropping when captured, and each of the items do something cool, maybe a permanent buff of some kind, or unlocking something new. Maybe each of the girls has a specific monster drop they go bananas for if you get it, a trinket or a trophy of some kind they hang/place in their rooms?
D. Some new achievements. Ideas: capture [X] number of monsters, capture one of each monster, gain a cumulative [Z] HP from capturing, have the sheep steal a cumulative [Y] HP, etc. Since we have to do all the achievements, we'll have to read them eventually.