I don’t believe working for a long period of time without income is a good idea. I certainly wouldn’t be able to do it, I’d be homeless in a matter of months. My understanding (which may differ from others) of contributing to open source software is that you so under the understanding that you’re not getting paid. Work a job, and contribute in your spare time. If you can find someone willing to pay you to work on it, then great! But it is unfortunate to lump genuinely beneficial changes to Cycles in with mostly unrelated (but still very valuable and well worth its price) learning material.
If you can find 100 people to pay a subscription to you in order to develop Blender for them, why not contribute those changes back into the community at large? The people still receive the exact same improvements regardless, plus, more people will find out and likely fund further development. I think that’s the essence of my suggestions.
I have thought long and hard about how to make a living from contributing to open source software. I have a few theories such as Patreon-style subscription based ‘employment’ where people subscribe to keep you developing, and an alternative feature-based crowdfunding style, where you put your price to develop a certain feature, then if you get enough backers, you do the work and get paid. Both of these allow the development to benefit everyone, which was the key prerequisite to my theories.
I haven’t considered what you’re doing as an option because I honestly don’t feel like it is healthy for the community. Noted, these are only theories, and I haven’t seen anyone try to work in this way, but I feel like there is a better arrangement than what you are doing now.