Personally been doing:

Participated in all 4 GNW is entry.
Must be a daily donor/high contribution. (More points if they actually do convoy/plunder/trials)
Rotate packs so no one gets legendary more than once every 3 weeks. (I keep an excel doc)
Personally ask whoever might be close to rare nin draw to get them their next strong ninja.
Prioritize stronger people with better packs.

It's a hassle and there's so many factors that come into play. I lead a 90 member group on S8, and last week only 2 were "unhappy" with pack distributions. You can't please everyone.

A lot of groups do the RNG thing too, but that only turns off more of the stronger players who believe they contribute more.

All in all, if they're going to leave they will leave. Try asking them their opinion on distributing- most of the time it's worse and only really benefits them.