A few days ago I started testing with the two SD breakout boards I got. One is a micro SD card breakout board, the other is an SD flash chip breakout board. I got the device tree overlay in place as well as turning on polling so it would check for a card inserted every second. When I boot the Pi up, it gives an error saying device timed out and unable to mount, but that's to be expected as I couldn't physically connect the card to the GPIO. When I hook up ground and power the Pi reboots immediately. I couldn't figure out why for a few days, but I have a theory to test now: Too much current draw. That could be a major problem. I don't believe I had the breakout boards powered directly from the 5 volt pins which can provide far more current than the 3.3 volt pins that were recommended in the document I was following. I have a 5 volt tolerant micro SD breakout board, so I'll start with that. If that works, I'll have a second SD card over GPIO and a working proof of concept for this project. That would be good enough for me to move onto software development. 

I'm going to post this quick update before I try to hook it up so I don't crash the system again. I'm running the latest Raspberry Pi OS (32 bit) from a Samsung Bar 32GB flash drive on a USB 3.0 port to avoid the corrupted SD cards I've been having trouble with lately. I had a 256GB SD card I was going to use for extra storage and backups on the Pi, but it got corrupted so badly I couldn't save it. Ordering a new 512GB soon.

I'll post another update soon with the test results.