Can anyone help me with ideas for driving a med/large dc spindle drive motor, please?

[Voicecoil]

To check what current the field winding will draw, firstly check it's DC resistance with a DMM, then apply the law of Mr Ohm to get the current - this will give you an idea whether you need to add anything in series for safety.
As far as PWM drive modules go, there used to be a guy from Hungary on fleaBay who made some good ones at a reasonable cost (around £100 or so) though I can't find him at the mo - COVID issue I wonder??? - we swapped a few emails and he knew his stuff. Otherwise go to Inverterdrive.co.uk, you will find a selection of drives there. The principle is just as you have described only bigger - you'd be talking of between 2 and 4 paralleled 600V+ MOSFETs top and bottom, or a pair of big chunky IGBTs. Provided you have a big variac there shouldn't be any reason why you can't test the setup at a lower voltage - and if you had a real monster you could even use it for speed control!