Just add a cable actuated disc e.g. TRP Spyre. Can't see why you'd need anything more in terms of messing around with the groupset. Most of the braking work is done by the front brake anyway.
But I'd need a new lever, presumably? The cable pull required for a caliper surely must be more than for a disc system? There is no understanding of science whatsoever behind this assumption...
Just add a cable actuated disc e.g. TRP Spyre. Can't see why you'd need anything more in terms of messing around with the groupset. Most of the braking work is done by the front brake anyway.