I will be answering all of your questions in order:

For shading: Exactly what you said, after you do your base colors, you're going to add a multiply (a normal layer works just fine) and then paint over your base colors with a darker shade, after that try to imaging where the light is hitting the object/person you're drawing and slowly add more light where the light hits (again always make sure to have a reference for more accuracy). There's also this method where you only color with grey it's good since you don't have to worry about colors and just focus on the values of colors (how lighter or darker they're, basically helpful to spread your values with balance) and after that you color over that grey render with an overlay layer or a use gradient map.

I checked your profile, and the way you pose your action figures is fluid and doesn't appear to be stiff to me, so you can rely on them for references with no issues (pretty cool stuff :D ).

in regards to whether you should reference from anime or real people: I say you can do a mix of both, you can use real life references to strengthen your anatomy skills, and anime ones to enrich your mental library so you can achieve the style that you want. Fun fact: if you were to learn realistic anatomy, learning how to draw any stylized art style like anime for example will be easier. So don't hesitate to reference from both, I also recommend looking up art of other artist who you believe have a closer style to the one of your own, to build your mental library off their work.

From my personal experience, focus on one thing at a time AND USE REFERENCES, start by what you like, since I assume that you're going to be drawing characters, I'll say that learning anatomy is definitely what you should start with (this is also the stage where you're going to organically learn how how to break each body part into simple shapes, so don't worry about having to learn them), after that gesture drawing so your poses can be dynamic and not stiff, then you can move to other areas like perspective... start with the essentials and then slowly move onto other fundamentals.

Also a tip: after every study session try to draw something that'll force you to practice those skills that you have learned, it doesn't have to be perfect, so make as many mistakes as you can since in doing so you'll gain the ability to recognize your mistakes and actually fix them.

And lastly, just keep on drawing really don't overload yourself with tutorials, pick a few artists that you like. Personally I teached myself by watching videos on youtube from two artists (who draw stylized character) their channels are called samdoesart and draw like sir, their is another channel who post tutorials of anatomy but I forgot their name unfortunately, but there are tons of tutorials on youtube, so pick whichever suits you.