Hey Ziggy,
you asked about using servo motors with custom controllers, however I know something about using them with the Buildbotics controller. The new Buildbotics Controller version has an additional 15-pin auxiliary I/O port which provides access to the motor enable, step, and direction inputs to the motor drivers. These lines provide the ability to bypass the internal motor drivers and control external drivers.
There was one user interested in using third-party servo drivers who reported having done that and used these external Closed-loop Stepper Drivers on the Buildbotics controller. For this, it was needed to modify the Buildbotics controller to lead the points mentioned above (step{X,Y,Z,A}, dir{X,Y,Z,A}, motor enable, fault) in the circuit out. A description of the project that user made can be found here. Finally the new Buildbotics Controller has the onboard connectors J1 and J2 on pcb / J1 and J2 on the schematics now which lead the circuit points out to the 15-pin auxiliary I/O port for connection of external stepper drivers.
You may ask, will this work with the Onefinity Controller, which is a hardware fork of the Buildbotics controller? Well the points in the circuit are there, at the inputs of the internal DRV8711 stepper drivers, however unlike on the Buildbotics, they are not led out. If you have the confidence to solder with an SMD soldering tip and to wire your own 15-pin auxiliary I/O port, like the user mentioned above did with the Buildbotics controller, then it would work. Note that the pcb layout of the Onefinity Controller (v4 (power rocker switch version), v5 (power push-button, self-powering off version)) differs from the Buildbotics controller’s pcb layout.
Further video watching
Precision motion control: ODrive Servo? Trinamic Stepper? Chinese Hybrid?