Since quite a while DuoMe shows the Leagues Hall of Fame (HOF):
It lists the total number of weeks you have been in the Diamond league. You may notice that number below your avatar in DuoMe/"yourprofilename". Adding a suitable number behind the above league HOF address: https://duome.eu/leagues/10000
brings you to a cohort of users with the same Diamond Seniority.
Those users are sorted by decreasing number of top-3 finishes. That may give an approximate indication of how 'league competitive' a user is. Approximate, because any user may finetune the probable average competitivity level in their league by choosing the league entry moment.
For the record: Over a year ago I got my legendary award with about 1280 XP. That same number barely kept me in Diamond a few weeks ago (as even the number 25 had a four digit XP total). In my cohort I am slightly below 'average competitive'.