When we installed the Curt Trailer Hitch Receiver # C12173 on a 2018 Buick Regal TourX in our shop we found it to have a ground clearance of approximately 9". You can compare this with what you have on your factory hitch as we do not have that information. This is currently our only receiver for your Regal. view full answer...
I checked the online edition of the owner's manual for the 2018 Cadillac XTS and it does indeed state the maximum trailer weight is 1000-lbs, just as you mentioned, but is also notes that the tongue weight of the connected trailer should not exceed 10% of that maximum, which would then be 100-lbs. This note appears on the right-hand side of page 233.
The three 1-1/4-inch Class II hitches offered for the car, such as Curt # C12115 shown on the linked page, all have the same rated maximum... view full answer...