Just to be pedantic: You've shown a number of power suppliers - not transformers (there's a subtlety in the wording there). I'd hold onto the 3A PSU for services and sensors, and supplement it with a new toroidal transformer for the drivers. Possibly hold onto the 12V depending on what it's currently used for, but it would make sense to rationalise the cabinet as far as is practicable, if for nothing else other than ease of maintenance). Your friend's suggestion of using two toroidal in parallel - broadly speaking, yes, but you're getting little benefit than using a single big one, unless you already have access to these? (or are looking to increase in future?, possibly just provision for the space of a future toroidal? - might make sense). I'm not quite sold on wiring two transformers in parallel that might have a different manufacturing tolerance - you could end up pumping some power from one transformer into another, but the power involved might be lost in the noise.

Your choice of 36V for the Z? - the original idea was to understand if the increased voltage improved your most susceptible axis. But that's your call and your game - leave you to it. I'd recommend replacing with the BFG toroidal transformer in the future.

Steppers with 6 wires?, measure the resistance between these - three will be (likely) connected to each coil - centre tapped, you probably should connect between the two wires of each three that has the greatest resistance (example you might find A-B = 1.5R, B-C = 1.5R, A-C = 3R, connect to A-C)