I'm wondering if that's the kind of thing you would be willing to integrate upstream.
As long as it still compiles with the Arduino IDE I see no issues with refactoring the code so it works better with Visual Studio.
I always wanted to make the code more modular. Like when you read through it you'll notice the same lines of code appearing over and over again, especially surrounding the file access. There is probably an opportunity hidden there to turn this into some sort of function.
I only finished the first 80 pages of "The C programming language" by Kernighan and it probably shows. I stopped when they started talking about pointers.
my solder job so far?
You need to set your soldering iron to a higher temperature since some of the solder points like on the AMS1117 voltage regulator and some of the GND pins look a little "dry".
If you have a spare PCB you can try different solder iron temperatures there and see what works best.
Are you using the newer lead-free solder or the old Pb60Sn40 type?
But overall it's fine and your Cart Reader most likely will work. Just do yourself a favour and before connecting the USB cable the first time check for a short between VCC and GND using a multimeter, better to be safe than sorry.
would it be possible to implement reading of the SD card through USB rather than having to remove it to acquire files? This would be a time saver.
If you can find an Arduino library/code example that can do such a thing and is not too big I can try to add it.