I love the form factor of the Switch also. I wish Nintendo hardware was more hackable.
On DSi you can replace the firmware and load your applications from the SD slot of the DSi, no flashcart needed.
Also, there are a few C/C++ toolchains for the DS (BlocksDS, devkitARM) and different libraries depending on how low level you want to go with your code. It's very easy to get started.
The 3DS is also pretty easy to hack, and then all you have to do is to copy your applications to the SD card of the console.
Also all Switch models can be hacked too. It's only the matter of softmodding (early V1 models) or modchip installation (later V1 models, V2, Lite, OLED)
I have both a hacked New 3DS XL and a hacked Switch OLED and both are incredibly good. But I admit I use the latter only because of piracy...
3DSes require a little more work and have a longer boot chain, but it's been thoroughly broken all the way to the bootstrapping process so you can use whichever firmware version and whatever patches you like with enough effort.
Proprietary closed hardware is not great for home brew projects. =)
The instructions on how to crack open the immutable OS image are readily available from Valve but you probably won't need them since it's already got a lot of power even without that.
I mean, I know there are a bare handful that can run Quake I. But I don't think there are that many, that can do more.
I do think it would be fun to have a plugin and be able to control lights from the Steam Deck menus, too, though. Just haven't gotten around to trying.
The decision-making here involved more passion than logic, clearly. And that's good. :)
In general, home brew people actually interested in building their own unique indie games often do not port to systems people have zero interest in supporting. Even publishing to Android/iOS would have less problems with <12% of users actually buying anything. =3