loose belt - or, perhaps they spilled a bit of power steering fluid on
the belt surface when flushing - that could also cause it to slip, make
the noise, and not drive the pump properly which would result in
additional steering effort.
Suggest replacing the belt with genuine Toyota part and see if you can
find someone to actually get the tension correct.
May not be as simple as it sounds.
I replaced my own a while back when I did the timing belt, and checking
about a week later, the belt was not sufficiently tensioned according
to the factory service manual.
Can't remember if I had to tighten it again after that.
Should be easy enough to install a new belt such that it doesn't slip,
although if there's residual spilled fluid on the pulley that should
also be cleaned to avoid contaminating the new belt, but actually using
a tension gauge to meet factory specs would be the next step. I suspect
most mechanics do not do this, but the tension in pounds is given and
also listed as a periodic maintenance item, so I set it to factory
specs. By the way, the alternator belt, which has to come off to reach
the P.S. belt, seems like it has to be really tight according to my
Burroughs belt tension gauge to reach the 90 or so pounds listed in the
manual, but if it is looser, and the car downshifts, revs up to higher
speed, you'll see quite a bit of "fluttering" in the belt, so I assume
Toyota has good reason for specifying belt tension in pounds rather
than "push it with your thumb and see if it moves apx. 1/2"  "

No telling what they put in there at those places..if you dont see any
fluid dripped on your belt, they may have used the wrong fluid.
I buy a quart of toyota ATF fluid at the dealer for a couple bucks and
do this myself with a large syringe. Go back there and find out what
they did. I would go to Toyota and have it done right and get them to
pay for it.