Youre basically asking for people who develop on open platforms for content on a closed platform. Buying a closed platform comes with its cons, and modding is one of the largest. (As very very few devs allow modding on console, and that list is almost essentially Bethesda, and not the Switch versions)
I just set my deck down to type this out. Lol. I usually assume someone would like to play on their TV and not a deck, even if i do 90% of my gaming on my deck anymore. You can hook the deck to the TV, but at that point you might as well get a $500 computer and a PS5 controller and have a better experience.
If you're just interested in modded Stardew, you've probably got a device capable of it already, or one can be gotten for quite a bit cheaper than a stream deck. Most computers can play it, the minimum requirements are extremely low, the only reason my last laptop (Chromebook specs running Windows, I'm a dumbass for buying it in the first place) couldn't do it was not enough hard drive space. The mobile port is also excellent, it ran great on my phone and I just tried it on my Kindle tablet and apart from some minor visual artefacts it's running well, there's options for controller support (never tried because my controller is wired and I don't have an adapter) and the Android version can be modded.