This is a fantastic set up, very fun to play out. No serious flaws stood out while playing, so just some quality of life suggestions:
-With its current state, the food mixing system could benefit from any combination you've performed before being marked in some way. The most simple version of this would be duplicating the "you've tried this already" message that comes from trying to re-mix a successful new food you already have, and just extending that to all combinations. The more ideal form would be when you select the first food, it visually marks all foods you already tried with it, or just prevents you from performing the exact same mix a second time by graying out options already used so you can't click them.
-A thought I had about the food system: Currently, once you've found your current surrogate(s) ideal food, the rest of the foods and mixing become pointless. If there's plans to expand the foods, an idea I had about them is that since they not only have a food they like best, but foods that would actually make them mad, you could expand on the idea by having each ingredient provide a benefit of some kind, such as needing to provide different surrogates with different balances of nutrition, and their ideal food doesn't necessarily provide all of it. Then, mixing and foods that aren't ideal, become more important because finding mixes of an ingredient they don't like that becomes a new food they will also like and provides the buff/nutrition they need/the player wants.
-Alternatively, Based on the menus, since there's potentially up to six surrogates being managed at once, with upwards of four more mechanics being added, I would say that unless there's plans to add more depth to the effects of food, you could refine its current state a few ways. I don't know/didn't test if there's a benefit to having them exercise every single day, or if it's only relevant for the small mood boost, (that you no longer need once you have ideal food), and for the debuff that you fix with it if you don't exercise them every few days.
Food on the other hand, as best as I understood the tutorial, is to be done every day (which makes sense), so streamlining it would be the ideal, because the player is going to be repeating it ad-nauseum.
Refining each food with a marker, like the mood smiley faces already used from the main screen, after you feed them it that indicates how much they liked/didn't like it, so it's easily seen at a glance not only what you've already tried, but what has worked best so far.
-If the food system is as robust as you want it in its current form, then I would recommend moving the button from inside Activities, to the main bar in the bottom left, for faster access since its done every day. Also, maybe adding the option once the player has experimented and found the best food, to mark a tick-box to always use that food, and then as long as it's checked, you just click the Feed button and it automatically feeds the current surrogate that ideal food. Dealer's Choice on whether it animates in the food cafe like usual, but streamlining the menu navigation on something that needs to be done every time seems good to me.
-I really liked the different chats the characters had for each exercise, and stage of exercise, and there were of course favorites I really liked. Once I had and was juggling all three characters though, constantly toggling away from each character made a few of the stages that have more subtle visual differences harder to notice when they've moved up a stage/tier/etc. While it doesn't necessarily need to be a part of the UI, a number or something similar somewhere that indicates what level they're currently at would be good, bonus effect if when you've reached the current maximum they can handle without a new upgrade, that number changes colors or gets a little " ! " on it to help keep the player aware they've hit a cap.
-Speaking of the caps, I think I may have min-maxed a bit too hard and completely broken them. I base that exclusively off the assumption that the 3rd cap upgrade is visually used at a certain size for Bess' exercise scene. I did notice that at higher medical upgrade levels they seemed to progress to larger sizes faster, so maybe that's tied to the expected child count? Either way, it would certainly add more 'umph' to the cap upgrades if the surrogates visually capped as well, possibly even the maximum child and therefore credit payout as well, really drive home that you're not developing any further without them.
That's all I could think of while playing, but I absolutely loved running through the game, and if I had to pick a gripe with a gun to my head, I'd say that I was a little sad when I realized I'd been wasting my time trying to find all the food combos, once I'd already had all 3 characters best foods, because every other food choice was then rather pointless.
