20 gl squat containers overkill? Maybe. Doing things in reverse, starting out with big containers to start, fill only halfway for the first 3 years, rootprune, fill container 3/4 for the next 3 years, root prune now all the way to the top.Come January 2021 complete overhaul, start over again.
Like their roots out into thick old hardwood mulch,finish with a thick layer of leaves. Not much watering from June till September.
When you fertilize your figs most will drain out the bottom that’s when the trees get most of the fertilizer. The outgoing roots will find it.