Hi @emdepqua,
For that many notes to be affected it is either a cable connection problem between the DM board and the GHL88M board that connects to the centre keys on the keyboard or a power problem.
I realize that you said that you checked for loose connections but did you check the connectors CN2 on the GHL88M board and the CN301 on the DM board as this is the cable that handles ''all'' the notes?
Here’s an image taken from the service manual, that shows where the notes are connected to the processor. I’ve highlighted the connector and also the power supply voltage. Although the power would be OK I think as you said some notes still work and it is the same power for all the notes.
[image|2343791]
(click on image to enlarge for better viewing)
Presumably there was no power at all connected to the keyboard when you did the repair?
If still no good then any reputable, professional electronics repair service should be able to fix it if you either give them the circuit or the link to the [https://elektrotanya.com/yamaha_ydp-s30_ydp-s30c_digi-piano_sm.rar/download.html|service manual]