This is a big part of the struggle with in vitro vs in vivo studies. We can have a repeatable result with in vitro effects, but can't confidently say what the macro effect will be. We might be able to say it doesn't "directly" cause none, some, or all of those, but we can't definitively say it doesn't contribute at all.
Personally I'm of the opinion that is evidence large scale studies are needed on live subjects. I'm not educated enough to know if that is feasible or reasonable, but I am confident you can't be "sure" until that is done.
There's effectively no way to test this with fully unadulterated control group. The entire planet more or less is covered in microplastics.
It's not clear how, even theoretically, you get better evidence than "this is what we see in vitro, and the effect seems analogous at population scale."
Sure, maybe there isn't, I can't argue that with my knowledge.
What I am confident on is there won't be significant societal change with that level of evidence. Most of those health issues have "easy" reasons they can be associated with (right or wrong), and it's going to be tough convincing people that in vitro effects are enough reason to significantly curtail (nevermind ban) plastics.
"Significant societal change" doesn't always have to come from getting masses of people onboard. It can also happen, and often does happen, by people entrusted with power to make decisions even in the absence of complete information.
One heuristic such a person might use would be, "gee, are we really going to take the position that if you pollute so quickly and so widespread that it becomes nearly impossible to demonstrate specific harm, that you can just keep on doing that?"
Personally I'm of the opinion that is evidence large scale studies are needed on live subjects. I'm not educated enough to know if that is feasible or reasonable, but I am confident you can't be "sure" until that is done.