A flow controller will save you water and resin, that's of course if you don't need 5 litre a min. I use about half that, and if you have been managing OK with a backpack so will you.
There is a cheaper alternative to a flow controller, either use a bypass system where any unused water goes back into the tank, or use a simple 12 volt motor speed controller wired in between your battery and your pump.
That is after all what the backpack speed controller does.