Unfortunately, I did not take any photos. I soldered the capacitors from +5V, +8V and -8V to GND on the underside of the PCB. Then I disconnected the plug for the 8V from the main board, put it through the ferrite rings (on the power supply side only 1x straight through the ring, the one on the plug for the main board 2x through the ring, otherwise the cable would have been too short) and reconnected it. However, as it looks in your picture, this is not necessary, as I can't see any of "my needle tips" on the signal there.
For me, there were always small needles of 2 - 4x the "noise amplitude". I got rid of them with this. The noise itself is still there and I don't know how to get rid of it. DavidAlfa has already tried a few things without success.
The spikes on my device also only appeared after some time, so I suspect that there might be a defect in the filter capacitors in the power supply unit or something similar.
A rising ESR would be a possible cause. However, the measured values of the electrolytic capacitors were normal.
For additional cooling, I use a simple 50mm fan for 5V, which I connect to about 3.3V (5V from the power supply, via 2x 1n4007 in series as a voltage dropper).
The fan runs so quietly that I can no longer hear it at a distance of 50cm.