Soft 404
That's great to see the 3rd harmonic way own into the noise floor!
Great update project. How big of an SD card did you use? I noticed the clip lead power connection, is this because of the battery's health or the lack of a proper charging supply? You might be able to do a quick and dirty USB charging, by stealing power from the USB connector and crossing over to barrel jack. Good luck with your future upgrades.
Playing a wav file with a 100 Hz tone :

Looks reasonable enough.

Since, in your words, it's in need of some TLC I recommend you start with this...
https://en.wikipedia.org/wiki/CrazySexyCool
I reviewed those types of adapters back in 2019 - they do work, although some of my readers do note some strange behaviour on Linux.
https://goughlui.com/2019/02/03/tested-generic-sintechi-fc1307a-based-sd-to-ide-adapter-sd35vc0/
- Gough