As mentioned before the reference motor driver could be powered by a Trinamic TMC4671.
This motor controller promises everything a universal torque mode motor driver should do while being compatible with 2 phase steppers, 3 phase servos and DC motors with the same power stage. Nice.
Big thanks to Trinamic MC for sending me a TMC4671 Devkit. This way i can begin testing the features before the fixed chip is sold again and the final pcb is manufactured.
And if you perfectly align the encoder speeds impossible for normals stepper drivers can be achieved.
The next step is to finish a pcb and wait until the TMC4671 is sold again. This might be the best solution fo a reference motor driver for this project.
Trinamic said to me that the TMC4671 in the fixed version is planned to be released in march 2020.
Until then the current version is sold that has a few hardware problems.
This means that the next prototype will still have the old version of the chip.
For SPI control and FFB the bugs should not be an issue apart from having to slow down the spi clock a bit for now to avoid a data corruption issue.
The step/dir interface does not work and the adc and spi MSB might have sporadic glitches which fortunately did not affect the motor control in the tests.
All known issues are listed in the errata of the datasheet.