A sharply defined line suggests to me even more that it's not an agitation problem, but caused by the developer not fully covering the film.(posted 8630 days ago)
As long as there's sufficient air space in the tank, inversion will completely mix the developer, using this 'infinity loop' method (never heard of it, sounds like new-age piffle), or just turning the tank upside down any old way.
You only get standing waves with continuous agitation methods, either by rotation or rocking.