Are you complaining about premades (le sigh), or just about stacking jobs? That's two very different kinds of topics...

Either way, I think your solution is the wrong one for many reasons: the queue of the duty finder doesn't discriminate between jobs for FLs, and it has always been so. In the past when the number of healers you had literally decided whether you won or lost games before the introduction of medkits and elixirs, people had to fight for who would sacrifice themselves to heal, every time. Every time, you had people fighting each other because the game asked them indirectly to field certain comps or not play the job they wanted and eat the bullet instead. What you are suggesting is not that different. You praise the ability to play the jobs people want (and I agree), yet you introduce factors that directly get into conflict with that specific idea. It is essentially unfair and scummy to let people queue up freely with the job they desire, and then have them be pressured to change their job once inside because the system allows you to do it.

If you want people to flock less to meta jobs, and limit AoE stacking, then I do believe there is other, way easier solutions that would make sense as well and wouldn't encroach into that principle. Else better to just have the matchmaker have role filters for each party like in pve... And I'm not sure if we want to get into that rabbit hole.

Reduce AoE stacking with diminishing falloff like in PvE past the first 3-4 targets or whatever works, really. That kind of nerf would be so drastically nasty already that it would shift the whole meta immensely.