All our Koi are currently residing in an 18'x4' above ground pool while we finish the expansion of our pond and they are loving it
. In fact the only issue we've had is fading Sumi on a few due to the pale blue interior of the pool. Your switch to EPDM should take care of that issue nicely.
We're getting by with much less filtration, but not feeding as heavily as you will be in a growout setting. The pool pump is being used with a skimmer for one Mat/LR filter while a sumbersible in the center feeds another. The water is maintaining exceptionally with very minimal water change needed.