I'll consider any suggestions, so feel free to express any personal preferences. Though I can't guarantee I'll accept them, I may be persuaded. I can always include a toggle filter in setttings for more extreme content that players may not want to see (such content will be lower on the development priority list however).
While I am no Zoologist, I do slightly dabble in it a bit. I'm currently touching up on breeding habits of things like Snakes, Fish, Spiders, Birds, and Horses to make sure my ideas aren't completely incorrect. (And making for a very bizarre search history lol) I believe some creative liberties can be taken as there is no of the monster girls have a defined species of their respective animal their non-human part is derived of. For Meru, I will be going aways outside the box simply because there is not a cannon version of monster girl that is part of the animal I will be suggesting.
On the Physical Side:
Mia: As you are going the live birth snake route, she is perhaps the simplest in terms of actual pregnancy, the breeding however...While normally snakes that only take a single mate only breed for a few hours, some snake will form *Checks Notes* breeding balls with many males around a single female. These can last days, and to cut out the middle man(Or men) in this case Lamia could have extremely long mating times and part of the challenge of breeding her is finding a way to survive the process.
Papi: I know this one is controversial because of her appearance, but cutting her out when she is one of the main girls would feel wrong (If she looked and acted a bit older she could have fit the "Flat Milf" archetype, which is honestly my favorite I.E. Milla Jovovich). Anyway here goes. Birds don't have much to work with, but Papi is dense, in order to court her you would probably have to impress her with dance and colorful, Flamboyant attire.(Courtship Techniques seen in birds.) It would add a comedy element to this part. As for the "Pregnancy" she is an egg layer, but since there is a human element to the harpy, I think there should be a change. We saw in the show how big the egg was, simply far too small to house a viable nearly human sized newborn. I propose that fertilized eggs in harpies need time within the mother to expand to acceptable size to support the growing newborn. So a rapid partial gestation before she lays a much larger egg. This means she will get to show albeit not quite as much, and will do so very rapidly.
Centorea: Horses are pretty plain as far as mammals go sexually. So while I can't find anything particular in actual zoology, I did notice something interesting on the Centaurs in verse. While it's pretty obvious the Centaurs genitalia are located on the back of the horse part, the lightweight Centaur had a very low cut waistband, to the point you could tell that were the front genitals would be would still be human and it made me think. What if Centaurs had two sets. One on their horse half, one on their human one. Double Pregnancy? It's rare but possible even in real humans (Albeit they are both located in the same area)
Suu: This one is tricky, but not because I don't have ideas, but because I have to many. There are so many options that you could go with. Ideally we all want to see preggo slime, so Mitosis would be kind of disappointing. Alternatively Suu is clear and while we definitely want baby bump Suu I don't think all of us want to see the...baby?...forming especially since some like me are going to want to sex her while pregnant. Slimes in many media have cores, I don't recall seeing one in Suu but that could be the bulb thing she has on her head. So instead of a baby slime inside her it would be a glowing ball growing in her, could even make a different color to represent the diferent genetics. When she births it she will give it part of her own body to serve as it's body. There is also there possibility that slimes, become Undines, since Undines give birth to slimes. So you may be able to go somewhere with that.
Meru: There are two possible candidates here, one is tame the other is not but more importantly both show. The first is similar to papis, unless you want Meru to carry her eggs in her mouth which some fish do. Might make for an interesting method of fertilization, via blowjob, but then no belly and that would be a shame. I suppose she could swallow the eggs after they are fertilized and instead of going to the stomach they go where they are supposed to only it's connection is only through the esophagus. (I just thought of this, btw so now there are three possibilities.) The final one would be...well...many fish are hermaphroditic, and we don't have a seahorse mermaid in canon, and damnit it's a guilty pleasure type kink you know. Basically you knock meru up, she carries half term, she knocks you up for the remaining. I wouldn't blame you for vetoing it immediately, but damnit, it's so hard to find and I had to throw it out there.
Rachnera: Many spiders engage in sexual cannibalism, which we don't want, not even on the dark path. After fertilizing she would also have to "Gestate" for a time, were both her belly and spinneret grows, one with the eggs and one with the necessary silk to make a sack. After which she spins them into a sac like normal spiders would.
Lala: Another one that is hard to figure out. This one is because I have no idea tbh. It's seems unlike Zombies and such, Dullahan in lore are not actually undead (Even if they were I would still want them to be impregnatable, If you decide to do smiths team I will think of uniquity for them as well, Zombina included.) This can honestly be the most mundane or the most interesting depending on how you approach it, but I will have to pass for now if I do think of something.
On the Behavioral side: We expect there to be drastic differences in behavior for a pregnant Monster Girl, the show itself highlights their normal differences anyway.
Mia: Desperate for constant warmth, this is typical in gravid snakes, Mia already seeks it but now she will be absolutely desperate for it. Likely spending a lot of time nude in the sun. Maybe could have a whole thing where she risks burning down the house in a desperate bid for warmth.
Papi: Obviously most birds nest, so hoarding behavior will be the bread and butter here. Did Papi already do that? I honestly can't remember.
Centorea: Restlessness seems to be common in pregnant mares, so extreme anxiety could be a behavioral change the MC has to deal with.
Suu: The sky is the limit here, but I think at least an even greater desire for moisture should be a good starting point.
Meru: This one I'm not sure of, lots of options, maybe constantly trying to stay out of sight/hiding.
Rachnera: This one should be a challenge, many female spiders will starve themselves to protect their brood, and never move from there location near the eggs sack. Also the spiderlings usually eat the mother...and each other which is a bit too dark. Seclusion and Loss of appetite could be challenges the MC faces with Rachnera.
Lala: She should flip entirely, going from her obsession with death to an equal obsession with life. It would be an interesting dynamic.
There is something else, I don't know if you plan to only do the main girls, or the main girls plus Smiths team, or what. That said once breeding is on the table Curie should be a candidate as the Vampires are on the verge of extinction and reproduction through pregnancy rather than through the "virus" would fix the issue (And pontential create a Dhampir species)
This is all just a very long winded spitballing of some thoughts, it's your game after all but you seem to be going really in depth, and that kinda stirred the creative side of me so I thought I'd share. Your free to use all of them or none as you see fit.
