Ian, I added a 3 axis DRO to my much smaller SX1LP. I purchased the appropriate optical scales and a head unit as a kit from Ali express. All up it came to just over £200, even though I made a mistake and ordered 1 micron scales. What came worked well, although it needed work to attach the scales to the mill – there was just a basic set of brackets in the kit so mostly I made my own.
On my little mill the quill action moves the motor plate and base of the head. I was able to connect the Z-axis read head to the bottom of the head so that it shows both Z axis movement and quill movement. The linkage and the head is not particularly rigid (but then it is a VERY small mill), so that If I lean on the head the readout changes 10-20 microns, but letting go allows it to go back. If you can engineer a link from the quill to The Z scale it is useful.
There is a bit of a write up on the Western Thunder site