The 19" stuff on SQ's remains internet echo as far as I know. I have not seen a single post yet where anyone has first person tried to fit either 19's, or especially 18's on it. Nor has there been an answer to my simple suggestion someone spend about 30 seconds and look at what size spare (the wheel diameter, not the tire) is back there on an SQ. That will be another obvious clue about wheel diameters. The suggested official Audi fitments don't mean those are the only ones or the minimum ones that fit either AFAIK
As I've said before, my D3 A8 with 385mm brakes of very similar design to the SQ's (slightly smaller at 380 mm) clears the brakes with factory 18's for that vehicle. My car has factory (optional) 20's but the factory spare is a baseline 18". The analog here would be baseline 18" Q5 wheels that are kind of dime a dozen available in the U.S. and could be good donor candidates for those who want narrow widths and higher aspect ratios for winter.
There is nothing magical about the SQ brakes beyond simple sizing and they use a relatively high volume basic floating twin piston caliper design, apparently shared w/ the D4 A8 3.0T by the way, and then with some black paint and a logo on an interchangeable clip. Perfectly competent design and found on other various higher power Audi sedans of fairly recent vintage and even up into some prior gen midsized BMW M's. They aren't at all like the bulky (and nice) many piston Brembo style used on RS4's and RS5's that do seem to require 19's even with only a 365mm rotor. The caliper shaping makes all the difference here and the newer style caliper designs in large measure are to allow rotor sizes to step up without extra big wheels, especially on the sedan/car side of Audi's products.
Someone please try an 18 and 19" factory wheel fitment onto an SQ5, or at least report on the spare wheel size. Anyone with standard Q5 wheels or S5 factory wheels would have prime donor test candidates.