Nice work! I moved one of those IBC tanks by myself once up a hill and it was a bear! Moving them by yourself is not too bad if you 'walk' it on its bottom corners but it is slow going. I had built a little roof (like 3 meters by 3 meters) on the hill and fabricated a gutter that fed into the IBC. The first time that it rained it filled it over halfway and it was not even a heavy rain.
A simple solution to keep the gutter from blowing around is to either screw or nail through the gutter flange and into the rafter. From the picture it looks like there is just enough of a flange there to do it.
I did not have the outlet adapter at first for the IBC port so I cut the bottom off a water bottle and used a hose clamp to attach the remainder of the bottle to the port. It worked super well to reduce the flow rate.
Sounds like you've got everything set up nicely.
I'll have a go at attaching the guttering - the problem is those are eucalyptus poles and are dammed tough, hard getting the pressure when you're up a ladder with one hand!
Nice idea on the bottom end solution too.
I did role another one up to another spot, it was too wet to walk it, kept churning into the ground!