My personal opinion is that Solomon had to have been in violation of Deuteronomy 17:17. The law is obviously a guideline, however, if having 1000 wives is not considered "increasing in excess", I don't know what is. We are not told exactly how many is "excessive", but again, I don't know how you can think that 1000 wives is not excessive.
This is obviously not the sin stated to be his downfall, but it is linked to it. My personal theory is that per Ecclesiastes, we see a man who is always reaching for more and struggling to be happy with where he is in life. Thankfully that book ends on a positive note, but it still shows his heart. From the biblical narrative, we know that towards the end of his life he started following all manner of occult practices- reaching for other Gods to give him the further knowledge and power that he desired. His excessive "wife-hoarding" seems to stem from his inability to remain content with what he had. He always had to be reaching for more, which is a great trait to have until it's not.
I think we can all learn some valuable lessons from Solomon.