Welcome to the forum! 😃
Regarding your question about body rotations in 2.5D style:
Here I show how to make an inverse transform constraint for the chest on Helvia: Twitch: Animating Helvia - loading screen by GungHo - Call of Guardians - YouTube (starting from 3:55:07 3:56:20 the constraint creation, I very soon add the additional bones I said I would not add, then change the weights)
Magnus chest: Twitch: Animating Magnus - Call of Guardians loading screen Pt. 1 - YouTube (jump to the end of the stream to see the final result) starting from 2:04:23 you can see the effect coming to life thanks to an inverse transform constraint.
I'd recommend having a look at this page, that lists all the streams, has some nice filters, and you can also search the chapters of the videos directly from there: Spine: Twitch
Shorter pills of rigging knowledge are to be found on this twitter thread: https://twitter.com/Er1k4r1/status/1035999798623711233
If you jump on a live stream, I'll also be happy to answer your questions and show examples live c:
And of course, if you post examples of what you're having difficulties with here on the forum, we'll try to help you out!