I got the 64gb and swapped in a 1tb nvme later on when I found a cheap one

the 64gb is an emmc in the same form factor as the nvme so probably doesn't perform much better than the sd if at all.

You can also always upgrade the SSD - it's just a specific type of m.2

Just to note, and by no means is this a common issue I've seen, but I recently had the SD card I had in mine (a legit Samsung card, 512gb) just randomly corrupt itself so all the games installed on it couldn't be seen by the Steam Deck anymore. That also lead to a bonus issue where the Deck thought the games where still installed whenever I tried to reinstall them and them not booting once they had been reinstalled, and at least one of them games (once I fixed that problem) still occasionally needs me to verify the installed files so it can reaquire one before the game will load

I have all my games on my SD card and still filled up the 64 GB internal storage. Yes you read that right. I guess shaders have to go on it (and OS)

I did this, and I have never had any issues. I have mainly been playing BG3 to be honest, but it runs great on the 64gb for me.