I am not extremely familiar with nix and flakes but from my understanding, they create isolated environments like venv or containers, correct? Would that not mean having a separate wine install and especially separate Lutris install per Game? So no single Lutris that lists all games but a separate one for each, kindof defeats the purpose I think. I fail to see the benefit over just clicking it on the lutris website, so not sure if this would gain traction. Open for any suggestions and corrections though!
Geekworm offers a HAT and an enclosure for 2HDD and the Pi, that might suit you