Fantastic stuff! There are plenty of modern flashcarts around for various systems, but most of them are designed to let you load up a bunch of (often pirated)

Fantastic stuff! There are plenty of modern flashcarts around for various systems, but most of them are designed to let you load up a bunch of (often pirated) games onto an SD card and play them, and they're not very good for development. Nobody wants to build their ROM image, copy it to an SD card, swap SD cards around, and then reboot the system. (Plenty of people do that, it just sucks.) It goes to show that making your own cartridge for development is not as hard as you might expect. There are some high-end cartridges out there that let you do everything, with both SD cards and USB ports, but it's less common to see pure development cartridges, which could be made much more cheaply.