(Maybe this will be my last post in this forum in my life)
1) SIGNAL TRACE = decoding ?
2) If you know the structure of the data patterns and servo for the specific brand, you will be able to decode sector data.
3) After that, you can rebuild the SA/MC... (to get the error list / skip bad sectors mainly)
4) ... and then rebuild sector by sector.
5) and then perform a logical recovery on the image.
You only need to know how data is recorded.
Heads are all equal in structure, drives do differ in hardware and mechanism i.e. head switching and preamplifiers, and data patterns on the platters.
Head switching and preamp are controlled by SW.
A solution for reading data is not complicated to implement , we have a lot of powerful and cheap (for me, maybe not for you) solutions that can do 90% of the work, the rest can be accomplished with pure software. Of course it will need a lot of work.
About head positioning, it is NOT a problem. I have discovered modes to successfully drive the VCM, to drive the spindle motor and reach rotational speed and to READ the channel (was an experiment on low density drive, but the principle is the same). With some software overhead and the use of simple solutions could be possible to implement a closed loop to lock the servo and make rotational speed stable (anyway my 3 phase driver was sufficiently stable - I used a professional power supply as source and the 3 pahse driver circuit used a quartz crystal as reference just beacuse there was no closed loop between the reading channel/index and the motor driver as in real HDD).
I didn't decode the data but this was not the purpose, I only wanted to see the patterns on the scope / logic analyzer.
For some models I have pointed out the head switching mechanism, for others not.
I know the majority on this forum only want (and is an actual user of) commercial solutions for work with drives, but maybe some of us interested in SERIOUS R&D (that has serious side effects like frustration, headaches, painstaking experiments and so on

)could join together to develop a non-standard solution that maybe can do what the other solutions can not. Anyway buying some equipment, click on buttons on a screen is easier than study, experiment and make your own system work so why worry ?