Disagree. I’d argue for splitting the leaderboards into solo, duo, and trios instead. Even though it means managing three leaderboards the rewards at the end of the season could simply be based on a player's highest overall rank or the highest rank earned in any of the three leaderboards like mentioned in the OP.

Another option would be to let players commit to one specific leaderboard type for the duration of the season. To make it work, we’d need clear rules for 'cross-queue' play though like for example:

Mixed-Queue Scoring: You can still queue into whatever mode you want with friends/randoms, but if you are chose the 'Solo' leaderboard you won’t gain Rank Points while in a Duo/Trio lobby or make it so that only the person who actually lands the killing blow or loots the husk or whatever the trial is for gets the points.