Interesting, I was just discussing this for my teenage cousin - we ultimately went with Deck but only because he needs to travel back to Germany.
I think your use case is different. Yes, the deck can play docked but much it is magic happens because the resolution is so tiny. Not saying you’ll be able to manage a great GPU, but you’ll likely get a little more out of a budget card at a standard 1080p. Also, with a budget PC, you can always upgrade parts if you’re strategic on parts like PSU.
As for building a library, you can always sail the high seas-- although there are so many budget friendly entertainment in the PC realm. Just refer to IsThereAnyDeal to ensure you’re getting the most bang for your buck.