I've been playing around with an esp32 SD card for similar retrofits: https://github.com/FYSETC/SD-WIFI-PRO
I actually have this flashcart. The issue with it is that it drains a heap of battery, so it's not ideal if you're just playing NDS stuff.
I also tried to fit the entire DS game library on an SD card for this flashcart, and the main challenge was figuring out how to organise the files so that it wouldn't crash the cart.
Basically you can't have everything in the one folder cause I think it tries to load everything all at once, so I had to separate it by a-z. And even then, I think I've limited to around 100 files per folder.
In the age of "Doom on a Pregnancy Tester!", I don't even click on this kind of stuff any more. Sorries!
Dreamcast memory cards (VMU - Visual Memory Unit) [1] were mini handheld gaming consoles with an 8-bit Sanyo LC8670 CPU and a monochrome screen. Some Dreamcast games let you "download" mini games into the memory card and play them independent of the main console.
[1] https://en.wikipedia.org/wiki/VMU