Mental health aid is squeaky-wheel based. It goes to those with the strongest advocates. Most mental health support frameworks are a patchwork of gerrymandered programs, jurisdictions and oversight that make the most of what meager funding they can get by obfuscating its availability and exhausting its supplicants.

The short answer here is your move is to contact the state, connect with your local social worker, inform them of your background and your history, and tell them that you wish to pursue guardianship of your brother. This will make you and your brother that social worker's problem. As you are a college-educated, sane human being that also happens to be vulnerable, the state's best move is to deploy as much aid to keeping you and your brother healthy as possible. They will not make it easy - you will be two of dozens (if not hundreds) of cases that worker is responsible for. However, they will be rewarded spiritually and professionally if they can keep you both in the win column.

You will not be rewarded with immediate guardianship. You may not be rewarded with guardianship at all. But you will become mental-health-adjacent, which in most municipalities enrobes you in some of the resources provided for mental health. If the state can more effectively care for your brother by deputizing you for his advocacy, they will aid your advocacy.

This is not the easiest path for you. Materially it may be the hardest. However, it sounds like you love your brother and I think you might find that the world is a better place for you both if you band together. It might even improve both of your relationships with your father if you can find a way to take this on in such a way that it does not belittle him or denigrate his own sacrifices. Find the easiest way forward, cast no aspersions and attempt to maximize the compassion in the situation. Social workers live for a success story because so much of what they do is finger-in-the-dyke bleakness. You could be that success story. Sell it.