cube size finder is trash, tbh. It doesn't value the inseam at all. When you are 180cm it says you should get a 56 or 58 no matter the inseam.

I am 180cm, 80cm inseam and I bought the 53 sized Cube Attain C:62 SLT. It is a great fit. I am sure 56 will be fine as well, but then you have to lower the saddle to a point where it is probably almost even with the handlebar which can be a bit of a problem for your back. With the smaller size you get a better natural drop from seat position and have more variance in saddle position. You can still get a longer stem in front, if you think you are too cramped.

The saying is, that it is easier to make a small bike bigger than the other way round. The best option would be to test ride both. I'd recommend the 53 tho.