I wouldn’t buy it to replace my phone. I want to keep that and my gaming devices separate.
I would buy it a dedicated handheld in a similar form factor to a phone + some controls. I’ve actually been looking at the AYN Thor for this exact use case. It would have to be the right price too.
Steam Deck can run Forza 5 okay enough, just don’t expect it to look amazing, especially while docked. It won’t be at the Xbox in terms of visuals and performance.
Based on how Motorsport ran on PC, I wouldn’t count on it being a smooth experience for FH6 until we are proven otherwise.
Also you can stream you main PC to a steam deck, although you’re going to want a wired connection if possible, especially if doing so while docked.