Hi @Demystificator,
I was like you in many ways when I first decided to start learning UE4 (5 months ago). I didn’t have much time but I wanted to learn the engine basics. I found it easier to start working on a simple game idea instead of just watching different tutorials. This basically kept me focused on learning all the major systems, but with a clear goal in mind (I didnt learn much about particles because my game didnt need them, but I learned about animations because I needed them).
I would say start with a template and add to it. So, for your Tomb Raider game, start with the thirdperson template and add basic features to it. My first game was inspired by Tomb Raider as well and I was able to learn the basics of the engine and create a 3 hour game in about 5 months. I started with the default TP template and added things like sprint, crouch, climbing, sliding, interactions, etc.
Out of all your ideas above, I would say the TR game is the easiest because many basic systems are already made. The basic character controls are built in. You can use Adobe Fuse to create and animate your characters for free. There are also tons of tutorials on basic functionality like edge climbing, swimming, grabbing objects, etc.
As far as your other questions, I would start alone and not bring in anyone until you have a decent understanding of the engine. Funny enough, celshading is actually harder to implement than a realistic style in the material editor (standard workflow is PBR). But there are several tutorials on making a celshading material on YT.
Good luck!