How to “buy” home from my father over time?

Looking for advice: Who could structure this deal? (Real estate attorney?) Has this been done before/what is it called?

My mother just passed and my father is headed towards retirement. He’s owns their home worth approximately 2 million, 600K left on mortgage with a $4.5/mo payment. He CANNOT sustain this payment (and everything else that goes into a home) in retirement and wants me to figure it out for him.

Plan: I sell my home, take the ~300K in profit, and use it to build an ADU in the back where my father could live rent and utility bill free. I move into the home and cover the mortgage. I would like to legally take ownership of the home day 1 (we are already co-borrowers on the mortgage, me + my dad). I would like an attorney to structure some sort of deal where I make monthly payments to my father (thinking $2K/month) to “buy” the 1.4 million in equity from him. This would give him additional $ because he will be living off of social security.

Me and my dad are 100% on board with how this will work. I have a brother that wants nothing to do with the living situation/figuring it out, but in the event my father passes before I have paid back the 1.4 million, I want the attorney to structure a 50/50 split with me and my sibling. Aka my dad passes in 2 years, I owe sibling almost $700K, if dad lives another 30+ years, I will owe my sibling almost nothing.

Does this seem fair?! What kind of attorney do I call to help with this?