Learn something new, but not sure I have time to make it this year.

Thanks a bunch for the guidance. I'm sure there's like a thousand things that could go wrong, or that I don't know, but I think this is good enough just as a head start. Hopefully I can piece together the rest eventually lol.

You use an existing mod as a guideline for what to do, yeah. I used the Garrus squadmate mod as a guide when I made my first one. The important stuff to look at is the Archetypes (What model is of type 'Head'?), SekeltalMeshes (The actual models), Materials (A set of properties that state how something it is applied to is colored), and Textures (2D images). You should also give a look into the existing mod's XComContent.ini and XComGame.int - they map Archetypes to body parts ingame and the ingame names of these body parts respectively.
Oh, and the reference skeleton is exported alongside the model itself. If you leave the reference skeleton out, you'll just import a static mesh, which won't cooperate with XCOM's animations.
This should become more clear when you look inside an existing mod's upk file.
And as a guy mentioned elsewhere, you should copy an existing XCOMHeadContent archetype from the game upk's and modify it instead of building one yourself.

Oh, thanks for typing all this out! I really appreciate it. I think I get the general idea, but correct me if I'm misunderstanding. Use the KK plugins you mentioned to export a model. Then use Blender to set up the animations, by using the reference model as a... reference. Then export the model (after splitting it up into different parts?) and put together the actual mod by taking an existing mod and overwriting its contents (which should be easier than setting it up from scratch). Does that sound about right?

If you want a female reference model, head into XCOM 2 Editor and extract SM_SldCnvMed_Reference_All_A_F.fbx. The model itself is only good for matching your model to it and little else, what you really want is the skeleton (armature).
If you manage to take your model - whatever it is - and alter it so it moves with that reference model armature, then you will have successfully ported the model.
Textures are a different beast. I suggest you rip any material out of an existing cosmetic mod and set the textures in there to your own so you don't have to set the parenting yourself.
And when you export the model from blender, be very sure you export in Face mode (NOT normal mode) and do NOT add leaf bones. Also make sure you export with all the vertices weighted to some bone you don't plan to use much. I for instance weight all my vertices to EyebrowUR3 with weight 0.05 and remove this weighting when I'm done rigging. If you don't do this the import could crash.

I see, thanks for the info. My experience with modeling is pretty limited, and the only thing I've done in XCOM specifically is a voicepack, so I'll have to do some homework on how to do the basics first haha. Do you know of any resources I could look at to point me in the right direction?

It is an involved process. You need the KKBP and CATS plugins to do it. You also need the patience to fix up the textures and remove useless vertices. And XCOM 2 hates transparency, so you'll have to chop up the model and get rid of all the transparent stuff. Other than that, pretty standard to how you would port any model over to XCOM 2.

I didn't realize it was possible to take a character from Koikatsu and eventually turn it into something XCOM can use. What's the process for doing this? How much work does it take? I'm kinda curious about trying this myself.