All of your criteria are excellent guidelines and aspirations. Remember, they are also relative and you may not always be able to honor your convictions with every piece and part of your home build, so be kind with yourself along the journey.
I'd recommend a local woodworking and metal working class, if you can find one. Learning basic skills (tools, safety, materials, cuts, etc.) in a hands on shop setting can provide a solid foundation for learning many other building skills.
I built a Skoolie my wife and I have lived in for 2 years. We learned a lot from YouTube and the Skoolie community on forums and social media. People are eager to share what they've learned. A bus isn't very modular, but many are affordable (ours was $4k), it's mobile, and there's strong argument for it being sustainable (it's reused, already has floor, walls, and roof). We used as many used materials as possible for the build and then bought new stuff for important things (plumbing, electrical).
Doing is learning. When we'd start a new part of the build we'd:
1. go to the internet and see what other people did
2. Develop a plan that fits our design criteria
3. Talk to a builder friend or local professional about specifics (code, parts, function, etc)
4. Start building
5. As challenges reveal themselves go back to the internet, friend, local pro, and figure out the details
Build your first house for your enemy. Build your second house for friend. Build your third house for yourself.