Down in our coastal area, when it was developed, about 18"-24" of topsoil was brought in on top of mostly sand. I can dig down an see the under-ocean of "beach." For the most part, the soil is rich, but the plants and trees compete desperately for it. My bananas, for example, will send runner roots more than 30' away. I will find them in our flower beds across the yard. I used to live in the red clay dirt further north, and it was much more labor intensive amending the soil conditions. I admire your efforts, as a trench that deep was no lazy man's task. You'll be greatly rewarded for it I think. Proof that we do this for the love of the trees, because I think often how easy it'd be to just order exotic fruits online, lol.
Back on topic, I would use those tanks for an aquatic environment--and am, with something similar. Throw in some aquatic plants mixed with hardy live bearer fish suck as Mollies or prolific guppies, and watch he magic. You'll need a good brand heater (like eheim) for this, but no need for filtration.
Benefits? Natural remedy to mosquitoes on your property. All mosquitoes will gravitate to it to lay their eggs and the fish will eat them, their eggs and larvae. Eventually, mosquitoes within a few blocks of you will be gone as they fail to repopulate.
Then, the container fills up with naturally accumulated rainwater and other elements. The natural ecology of the water is perfect for use in watering your tropicals that are sensitive to burning from chemicals or other side effects from tap or other sources. Ad someone else mentioned, you can also build an overflow to capture and remove excess to be used in drip irrigation or other methods.
But most of all, you can grow some rare tropical water plants and own a large number of fish that you can cull and feed to your home aquarium predator fishes, as well as filtering out some water and media to shower some plants in rich liquids. You might suck out a few fish in this process-'but makes for good fertilizer IMO