Good progress man,
Here are some of my thoughts regarding rigging:
I dont really like that animation but well, but hey, we cant expect the world from a free tool.
You should not use this motion to actually test your rig deformation… it just doesnt have a wide enough range to reliably tell if stuff works or not.
As far as joint placement goes, it doesnt look too bad, quite hard to see it that well with the animation going. But I think the ankle joint is a bit too far forward, have it more central. The clavicle base needs to come a little more forward, it should be placed anatomically so right at the actual bone you can see. The shoulder joint should come up and in a little more, take some looks at actual shoulder references to clearly see where the joint’s at. You could use one more joint in the chest, or redistrubute the existing spine chain. Just to be able to get a little bend going on in the upper torso as it shouldnt be completely rigid - especially on a stylized character like this, otherwise itd be a little hard to get certain bowing poses.
Hip joint looks fine from the front, cant quite tell about the side, a still orthographic view would come in handy here.
Regarding your deformation concerns… I dont think muscle is an option on a character like this, also considering your experience level in rigging it would probably be quite a process you wouldnt be willing to go through as you said rigging isnt actually your focus of interest.
Corrective blendshapes might do, but I find them quite unflexible and as Nick said, they should be only a final step if you havent got the time to solve stuff properly. The issue with correctives is when you use them to correct big chunks of foul deformaiton like arm/shoulder up and rotated forward (i.e. reaching to the back with the hands from above). Or even just elbow and knee fixes when the limbs bend sharply. If that deformation is too messed up from the start then a corrective might solve it in that exact pose, but in the blending process youll get huge mesh shifts and bloats in unexpected directions. In my opinion corrective blendshapes should be used more to add an extra layer of detail to the character by adding muscular action and volume shifts with them and not really to solve bad deformation.
So in short: corrective shapes (unlike the name states) should be more for improving stable deformation, not to correct sh*tty deformation.
About the upper torso, and back deformations on shoulder movements, I would add in some extra slidy stretchy joints to fake pectoral muscle range on the chest and scapula sliding on the back.
You can use a setup like that on a lot of places to introduce “nonlinear” deformaiton and volume preservative features.
Also having joints that move out as limbs bend can help you get a nice shape without the need to use correctives (even tho the stup might be slower, in the end it pays off as the deformation is way more stable when blending in)
Extra thouht: see what you can do on elbow and knees, maybe dual quaternion can help get the shape preserve volume, however I would not recommend it since DQ doesnt work very well when scaling joints non-uniformly (stretchy limbs and whatnot) also it will probably mess it up if you actually do decide to use correctives later too (unless you are doing correctives directly manually in deformed mesh space (thats messy) or there is a mesh delta extractor algorithm around that also accounts for quaternion weights - never tried it… anybody knows that?)
On the breathing you could get away using just animated keys on control offsets as Nick suggested, using some fancy pre-multiplication setup and maybe a simple expression, animCurveUU or remapValue node rig to tweak per area intensity and rate of the breath anim, also maybe a little blendshape for volume shift from chest to belly and vice versa…
As facial is concerned, I guess youd be best to go with a blendshape driven rig for most parts… maybe save the jaw opening as thats better off using a rotating jaw joint. Then maybe a small corrective on top to have some skin slide on the sides whilst the jaw opens (just throwing in some crazy thoughts there). However youve always got the option to have more of the broader facial stuff be deformed by joints, and use correctives to get the deformations to a nicer level - I prefer this way…
The problem with blendShape-only rigs is that its quite difficult to have the shapes be compatible with each other while still have a large range of effect to not have the feel of the facial sections being detached from each other… the whole face should feel connected as its one big network of muscles acting together and one big flap of skin lying on top of it.
Can also discuss dynamics as you progress the rig…
Anyay dude, youre doing good work already, good model and textures, and the renders look solid.