oh hey thank you so much for the feedback. its not often i get some so pardon my late reply.
i agree the shrink ray is too expensive i think. especially since i barely play survival and still never killed the wither despite playing for over 10 years. when i was making the recipe, i was not sure if i wanted to use a diamond or something more rare. i think i want something more rare but not as rare as the nether star.
as for the internal layouts, i knew about the cow fun fact :P but TBH, i just got kinda lazy with those. making new stomach models and programming them in one by one is a huge pain in the ass. i think there is a way for users to do customized stomachs for mobs without editing the mod code but i forget how and i never got around to documenting it.
as for other orifices / vore types, that again comes down to laziness and lack of interest on my part. but again it could me done manually to mobs by creating a custom stomach and modifying the NBT. the message template as well as the type of stomach a mob has is stored in the NBT so you could set a cow to have a custom 4 stomach model and set specific mobs to unbirth you by teleporting you to a special womb "stomach" model and have the text something like "{prey} was unbirthed by {pred}"
i've wanted to document that but i've procrastinated doing it for sooo long because i have no interest.
and because of my lack of interest, feel free to fork the mod and make your own changes and see how it works under the hood. since its open source, you can do that and don't even need to ask for permission :P just make sure your modified fork is also open source. if you make changes, i would be excited to see them. maybe if i like the changes, i might add them to my version of the mod. but maybe not because i reckon that would be a lot of work on my part. i guess will see...