The dot would indicate pin 1 Colin, sometimes it’s either a notch or dot and sometimes both if it’s both you’d go with the notch
Can’t really see the component side pictures they’re really dark with the shadow, we’d need to be able to clearly see all components, resistor colour bands and your offboard wiring so that it can be traced
Your TL072 readings are too high VR going into pin 3 should read half your power supply check what voltage you get at the junction of R17 and 18 the power supply voltage divider you can use your meters continuity check to see where they join
What are your voltages on the PT2399 pins 1, 2, 3 and 4
Pin 1 should be around 5v with 2 half of pin 1s voltage and 3 and 4 are grounds or 0v
I think looking at your solder side R22 appears to lack solder I don’t have the build doc for the tap tempo which I’m assuming is your other board so don’t know what you leave out etc
*EDIT just found the tap tempo doc and see the notes on R22 so ignore that!