Yeah, I'm not totally sure it could be implemented right now anyway. I know that the ESP32 can apparently be updated over the air, but I think the ESP32 is also more powerful than the NRS-52 in that regard, because it has Wi-Fi and other stuff like that.

I think you're right in that there would have to be a way to load a firmware from an external storage such as an SD card and your upload would just replace that firmware and then you would tell the system to reboot and load the new firmware from the SD card. Either that or as you mentioned above have two MCUs.