2021+ K5 body is all the same. Only difference is the engine, bumpers, and accessories. You can go with GTL, EX, or GT wheels without modification. Just stick with 235/45/18 or 245/40/19
20s will fit on stock suspension. You risk going with a thinner tire and compromise ride quality
20s will fit on stock suspension. You risk going with a thinner tire and compromise ride quality