As the sound happens under braking only i'm of the opinion its brake based.

Is it a pad squealing? almost all cases of squealing in my experience have been high speed vibration of the metal backing plate of the pad against the caliper, could be where the piston contacts the pad or the edge of the pad vibrating against the caliper itself.

Do you have those nice little sprung stainless steel clips on your calipers, clipped into the caliper and the pad slotting nicely held in by that clip? i ask because i've found that over time crud builds up between the caliper and that clip, i ping them off clean the clips up with a wire brush and scrape the caliper clean where the clip sits with an old chisel or sharp screwdriver and finish off with wire brush, sometimes when you find new pads awkward to fit after cleaning up like this they slide in easily so able to retract better after the brake is released.

No doubt your indy cleaned and lubed the sliders up well during refit, another thought is the back plate (assuming one fitted) so close to the disc that under braking its catching.

Is the tyre far enough from the suspension strut, you should be able to feel that gap yourself even if unable to actually see even on full lock.