[dragonwriter]

> But you still need to work around collusions between candidates

No, you don’t, if it’s a multimember proportional system, like STV. There are some multimember systems your concern applies to, like the “vote for N, top N win” at-large system used for some US city councils, school district boards, and similar bodies: those are simply means to have a multimember body while avoiding either diffuse or geographically-concentrated minorities from being represented. (And the prospect of that particular kind of multimember arrangement being used to neutralize the effect of black voting rights, specifically, is why multimember districts are prohibited by federal law in Congressional apportionment, currently.)