I see that several players who smurf say they do it because their friends are a lower rank than them, so they make an account and get it down to their rank.

I would suggest that Riot ignore rank disparity in a party only in the case of 5 stacks, then enter competitive matchmaking with the system treating all members of the party as the same rank of the highest ranking member.

For example, say there is a party of one Platinum 3 and three Gold 1. They would not be able to search for a match. But then one more Gold 1 friend comes along to complete 5 stack, so they have the one Plat 3 and now four Gold 1. Then the matchmaking system would treat them as if they were five Platinum 3 players, and accordingly giving them opponents in the range of Platinum 3.

This way, groups of 5 friends could play competitive if they don't mind having tougher opponents, perhaps removing a bit of incentive from smurfing in this situation.