Nano filtration is similar to RO, but less expensive, has lower water loss, and less complete filtration. In a pool situation, the main problem with both RO and nano filtration is water lost keeping the membrane from getting fouled/scaled with calcium and other minerals removed from the water.
If I am following what you are trying to do correctly, filter out the calcium from your fill water as you refill the pool, you might lose 25% with nano filtration, compared to 50+% with RO. Nano filtration clearly beats RO here, but that kind of water loss is still fairly large. That water loss might be acceptable to you, obviously that is a personal decision, but I don't see the point. Occasional commercial RO treatments (which use large and expensive filters that are much more efficient that what you could build for any reasonable price) are most likely to be significantly less expensive in the long run.