Out of curiosity, I just looked on
www.rimstyle.com website to see what sizes of alloys they display for your car and the smallest size they return in the search results is 18” (see screenshot below) - presumably, as you’ve said, because of the Cupra’s larger brakes.
Even if some alloy wheels would, in theory, fit over the brake calipers and discs, there may still be clearance issues between the brakes and the stick on wheel balance weights. Certain combinations of wheel width, offset and spoke design could also present clearance issues (e.g. the reverse side of the spokes fouling the brake calipers).