I’ve been playing BG3 on my SD card for a little over a month now and yes, it does work technically and if you absolutely have to, go for it. That being said, this is one of the only games I’ve played where the difference is more than just a second or two more per loading screen. I had assumed that the issues I was having were just due to general Deck limitations but I changed over to the Internal storage last week by moving basically every other game to my SD card and all of my issues are gone.
The launch failures and the glitch Id get where only half the textures would load and characters were missing torsos and the map was see through. The extremely long load times for initial boot. The crazy long upload time for cloud saves. All gone.
So go make room for BG3 on your Internal if you can, it genuinely makes a huge difference.
What brand and size SD card were you using?
I think this is true for quite a few modern games unfortunately - it’s quite problematic with only 1TB of NVMe storage.
Ratchet and Clank is a really obvious example, but there are quite a few others. Starfield ran much better from NVMe for me, modern Forza games, Cyberpunk and CoD also. As you say: It’s not just loading screens but texture streaming issues and stuttering too.