INA155 Instrumentation Amplifier
I tried replacing the INA128 chips on the Electrode Module 3 with INA155 chips with a Gain of 50, and it works just fine :) The signal even looks a little bit cleaner than with the INA128, though I don't have good metrics to decide which one is better overall. They both have the same PCB footprint so I didn't even have to change the board design :)
This is good news. If I buy the INA155 chips in bulk (250+), they would cost ~1.75€ each (and I currently pay ~5.65€ per INA128), which would bring the material cost of the whole product down from ~60€ to ~30€ plus Arduino (~38€) plus shipment. Even if the INA155 is slightly worse, that's totally worth it.
At this price, it would even be affordable to use more than 8 signals (with additional analog multiplexing, since the Arduino only has 8 analog inputs) and wear more than two Electrode Module 3 at the same time.
Another big plus is that INA155 chips can easily work with a supply voltage of 3.3V!! And I wouldn't need to boost the battery voltage all the way up to 5V anymore, which is probably not very power efficient.
I did some more tests and concluded that with the electrodes that I'm currently using, the maximum gain of INA155 is not high enough, and I have to either find better electrodes, add extra amplifiers, or switch back to INA128 for the time being. For now, the relatively expensive INA128s will have to do.
But hey, the current electrodes are literally just... plain metal spacer screws, so there is DEFINITELY room for improvement.
Here are some pictures of electrode placement and resulting signals, where you can see that the INA128 (blue line) shows considerably more features: