I do not think there are many microbial benefits to a DSB, there's just no need to wastr the space ina FW aquarium with plants.
Plants are far superior to export than bacteria ina DSB.
DSB are useful for marine aquarist who wish to save $ on salt cost for water changes. But here again, the marine aquarist could use macro algae or mangroves/emergent species, seasgrasses, but most find that the macro algae do the best job. In those cases, the micro invertebrates and macroinvertebrates form a lot of close relationships not found in FW systems.
You can also use micro algae, search ATS, algae turf scrubbers. These are better suited to smaller NO3 and PO4 export and can handle more abuse and ranges of NO3/PO4 than macro algae can. But macro algae can export more volume of the ferts per unit area and per light PAR.
One way to test this is to remove all the plants and add KNO3 and see how much is lost over time.
Plant roots add carbon and O2 that will help some bacteria, but the denitrifiers should be okay without that source.
You'd need to do this a few times, say 4-5 weeks. So this is a better method for a FISH only tank, not one with plants. You start havign a very hard time figuring out who is doing what if you start removing NO3 via bacteria, water changes, plants, macro algae, mangroves, clean filters etc etc.
A simple system works best and then you can later add treatments on top of that.