The whole role lock thing is stupid, BUT I have a solution.
Unranked: Keep it casual
Ranked: Minimum 1 of each role, maximum 3, allows for many comps and keeps it stable.
the issue is that you're introducing role queue in a slightly different way, it's still "role lock". not to mention that I'm sure certain compositions will be better than others as people learn the game so your idea won't be good anyway, maybe you'll have a team playing 3, 2, 1 and another team playing 1, 3, 2 and the latter is objectively superior (just a random example, I have no idea how the meta will develop).