I could see ideally a chemical self destruct timer like "it becomes biodegradable in 30 years" but that is clearly easier said than done given issues like reaction rates varying by temperature.
Look up how ethanol fermentation is done -- it's actually not that sterile. The yeast secrete alcohol that effectively kills off competitors. A similar effect has been taken advantage of by engineering some yeasts to consume an unnatural source of phosphorus (phosphite).
So if you were to evolve a microbe that can eat certain types of plastic, it would readily outcompete everything else.
Although even if the bacteria itself is innocuous the byproducts may be less than healthy - alcohol isn't exactly great for humans although it is widely enjoyed and well botulism is far worse and I expect half-digested plastics to be less than healthy even if they are chemically inert.