I got the 64gb version in batch one, and immediately upgraded it with a 256gb ssd I had laying around from another system. It was pretty easy, and flashing the OS was trivial even in those early days, but I did find it difficult to get the EM shield to go onto the drive that I chose... It was a tight fit and took 10 minutes of tinkering to get it on there.
It's turned out to be nice to have the extra internal space for doing things in the desktop mode (especially tinkering with emulation) just because, while the sd card is seamless for steam libraries, it adds some complexity and headaches for other software.
I'd imagine that getting it to run on deck will involve some major sacrifices to both resolution, and to things like draw distance that will push the CPU too hard. The problem with the deck for some AAA games is that it struggles to push both its CPU and GPU at the same time because they share a power budget.
Tod Howard mentioned in an interview with IGN that the game could run well above 30 on console for many areas, but they capped it at 30 because some areas (presumable the large cities) are much heavier. My guess is the game will run fine on deck at 540p 30 with low settings in many areas, but will be pretty rough in the larger cities. Who knows though, it may be much more scalable than they've let on with the minimum hardware requirements.
The other thing to keep in mind is that since it is still based on the Creation engine, there will surely be performance optimizing mods that remove ground clutter and other cosmetic assets to improve performance soon after launch.