> Make the plastic manufacturers own the external costs by requiring they fund proper disposal sites/messaging,
I think you fail to understand the reality of the problem.
In western countries, plastic "trash" is not really the problem. It's highly visible and it would always be nice to reduce it of course.
The majority of uncontained environmental microplastics comes from vehicle tires and clothing/textiles. Clothing and other textiles (e.g., carpets) being the biggest source, more than 1/3rd. After that it's probably building materials, paints, machinery and factory parts, etc.
Disposal sites and messaging will not do anything. You can be a perfectly compliant goodly consumer who dutifully puts their old clothes in the trash and pays the disposal fees for their old tires or rides busses. You'd still be contributing enormously to environmental microplastic load.
All natural fiber clothes, cycle everywhere, don't wear sneakers or other kind of plastic or synthetic rubber shoes, don't have synthetic carpets or drapes, don't paint your house, etc... now you're starting to get somewhere.
But the machinery required for you to stay alive, moving goods and services around, pumping your water, people going to work to keep your electricity on, package your food, etc... all still pumping out microplastics.
Disposal and messaging just won't cut it. And without a bunch of astounding and vanishingly unlikely breakthroughs, getting rid of microplastics from the top 4-5 sources will make net zero CO2 look like a walk in the park. Therefore we have to accept microplastics at enormous scale and work with that. Not to say we shouldn't attempt to reduce it where possible of course we should, but it won't be reduced to insignificant. So I think what needs to be done is well funded research into the effects of existing and new types of plastics, and into new materials and techniques for cleanup or containment. That way we have a chance to discover and limit or ban the worst of the worst before they can become too pervasive.
As far as reduction goes, possibly some small incentives to avoid plastics in consumer items (clothes, carpets, etc) might help. The messaging really can not be the same idiotic and counterproductive alarmism and blame and guilt campaigns led by wealthy private jet and mega yacht owning billionaires of the climate change debacle. Just gently make people aware they could look for natural fiber clothes, perhaps modest and commensurate added costs on plastics manufacturers to fund this research and containment, etc.