Sounds super complex to approach this way, and then keeping the genes up regulated in a single plant for all of that functionality over time and survive? Not so certain. To dive down that rabbit hole, look up known phytoremediative transgenic projects working with defined plastics, as your system would need to be a bit specific on the medium to degrade to start off. Sharon Doty at the university of Washington has some good papers on this, mainly looking at other environmental toxins like TCE, chloroform, TNT, and benzene.
Would probably be easier to approach from a guild planting / polyculture perspective. What other plants can provide a step in the degradation? There are known worms that eat polystyrene in cycles, as well as others for other plastics. You can find a video on the Thought Emporium channel on YouTube. Then the question, how can you create redundant degradation steps in the system, that simultaneously benefit the other organisms so none of them die or get too stressed?
Like in your system, you note plastic as the roofing material for a slurry of microbes and fungi, with a plant. Question 1) what are optimal ratios of waste : medium? Do a spectrum of experiments in steps from 0 : 100 to 100 : 0 so you can learn how much, say dirt, or mix of dirt and mulch or whathaveyou you need to offset the plastics in the system. What are the limits on the system functionality?
Once you have established boundaries, then is good to start maybe thinking of getting into the transgenics, more likely adding more organisms, and general complexity cause then you can characterize what’s in excess in your system, to then search for metabolic pathways, to then identify enzymes, to then find the genes… to amplify, clone into a vector, transform agro… whilst tissue culturing a plant, developing an initial assay, testing it transiently… then testing your construct… or constructs. It’s just a lot of work if you don’t have some benchmarks yet…
I’d start with a bunch of plants at a similar stage of growth, mycelium in long phase growth, whatever bacterial slurry amendment you’re thinking, and a mix of dirt and mulch and just see what percentage (g of plastic per g/kg of biomass) of defined material your system can take before it all dies. Also… maybe look into how to clone or further propagate those strains as they develop, as you’re essentially breeding new ones and may wanna keep a library.