If you want to make sure plastic never gets into the ocean, disposing of it properly should do it. If it doesn't, something is wrong with how garbage disposal is done and that needs to be fixed.
There are also some unanswered questions here about where microplastics in the environment actually come from. I would expect litter to be a much larger source? How about fishing equipment?
I would expect stuff in a landfill to be less likely to get out than any other way. It seems like an easier problem to solve?
To be honest, I think micro-plastics are the among least hazardous things that can leak from a landfill. Also a landfill is usually local as possible and doesn't require much energy.
The answer will always be "Use less of the junk." But I feel guilty putting plastic in the recycling bin.