I partially got persistent_ram working! Only problem is, it doesn't seem to want to read its data at the moment.
== 26-12-2020 ==
Got a new microUSB plug breakout board (finally I have a proper cable that doesn't break apart), but even then the results were the same. I tested the cable on another device that should have working UART, but to no avail. I think my serial board is broken. I tried using my raspi for debugging, but didn't get it to work - uart via gpio is incredibly confusing, and it doesn't help that the raspi is slow as hell. Oh well.