A primary condition of doing anything to the animations is that they can't be lower quality. Slicing while not having zero impact on the image quality (a few have slipped through with odd gaps, but when I find them (or people report them) I fix them) it does let me put them out as a non-compressed version (vs the 5.8GBs they were before slicing).
I'd love a similar thing for animations, an uncompressed version for people (currently ~7GB uncompressed for the videos), but alas, I fear it is most likely never going to be the case.
I'm not sure how photoshop editing would really help here. I already have several video editing (and image editing) tools at my disposal. And I can't make the animations smaller as video (while keeping them the same duration, if I make them all 1 second long, then sure, I can cut the size down to around 100MBs but then they would be crap).
The problem isn't getting the animations, or even their parts, it's how the animations are played back by the game.
Right now they are video files, which, for all the reasons above, uses a lot of disk space, that I can't really make any smaller. Even with video compression (and additional compression on top of that), they are pretty big. But, if we look at just the parts that make up the animations (the files the Unity game uses) they're only around ~170MB. The Live2D plugin and Unity Engine put all the parts together in engine, and render the animation in real time.
This means Unity Engine only needs the animation model parts, and a data file describing those parts, to play them back, files that are tiny in size. I don't have those data files (not the whole files anyhow), and even if I did, what I have access to isn't compatible with Renpy.