A good layout is one that meets or exceeds your expectations (or design specs). These specs form what is called Design for Manufacturability (DFM) for Testability (DFT) for cost (DFC) and for performance of accepting signal inputs by quick connection, signal conditioning for gain, bandwidth and CMRR (f) with tolerance errors and outputs. The layout is expected to not cause EMI pickup from handling , use a convenient power source, single or dual supply.

The existence of measurable design specs is essential before starting a design and changes are allowed then these become the criteria for Design Verification Tests (DVT) with a plan on how to test each parameter.

Once you learn how to evaluate existing commercial designs and not academic DIY designs, your powers of observation may show you how to meet all your requirements. Ground noise and "Right-Leg-drive" feedback is one thing you seem to have overlooked not using R5/R6 midpoint floating for feedback with a buffered RLD.

@TonyStewart thank you for the tips. The driven-right-leg technique is used to attenuate common mode residue by an additional factor K, after the differential signal has been sent through the inamp. I don't use a DRL-circuit in this design, because I post-process the data with a moving-average filter to remove remaining common mode signal. I have also found that turning off the lights and unplugging all electronics from the wall helps tremendously in reducing noise when measuring the ECG.

@Carl That's good. It doesn't have to be the right leg either. It could be the hip or some other body part that doesn't move much. Then you can see what you are doing with the lights on ;)