I hope this isn't too difficult, but a lot of people still play 1.12 specifically, and support for that version (though I know is not planned), would be much appreciated.

I don't think the mod will be ported to 1.12 because of how marginally different that version is in its code, on top of it being an old version and Female Gender being a new mod, so I suggest you could use More Player Models which already has an option for breasts (tho, no scaling for them, you just pick between 2 or 3 presets). I guess your satisfaction depends on how customizable you'd want the mod to be.

1.12 seems to be a version that a lot of people stay on, as it has a lot of amazing mods for it that were never updated. I was just trying it out but I think it would be greatly appreciated by many if you did

1.12 is not happening officially. If someone ports it themselves to older versions I'm happy to share it, but I won't be doing it myself.