Based on the picture in my initial post, it looks like I was getting somewhere in the vicinity of 0.7V amplitude, and my hall sensors came from aliexpress if it makes any difference.
Could you please post some pictures of your setup, or provide links to the motors you are using? Otherwise, the best feedback I can offer is that it is very likely that the back-iron on your motors is too thick to let any magnetic field through as you suspect. It is fairly easy to tell if a motor will work by checking whether something metal will stick to the rotor shell with any substantial force, and about half of my motors won’t even attract a paperclip.
I have also looked at the second sensor orientation with the oscilloscope, with the halls under the motor lip, and the waveform looked pretty decent to me. I think the magnetic field from the magnets dominates the magnetic field generated by the stator coils, but this is just anecdotal for now. The challenge with this sensor placement is the sharper curvature of the magnetic field in the area under the lip makes it much harder to achieve optimal positioning and signal amplitude.