There are plenty of supposed Steam Deck killers available in the portable gaming space, from the Asus ROG Ally to the Lenovo Legion Go and all the Ayaneo and ONEXPLAYER devices in between. But none of them have managed to outright slay the dragon that is the mighty Steam Deck yet.
Ultimately i think its just a game of optimizing CPU. Thr steam decksnweakest component reletively speaking, is the cpu, given its zen+ based and not amy of the newer generations. Starfield, reletive to other AAA games, is fairly CPU heavy. Given its likely going to take the same mod direction as fallout or skyrim, itll eventually be fixed and there will likely be some veey basic modpack that optimizes specifically for the deck.