Perhaps "one up" your left wing is a touch low and it cross feeds?
I've not seen an aircraft (of our type) flow evenly from both tanks. Even the expensive Euro types.
Whether it's flying out of balance or a wing low.....or poor venting.
I do "manage" my fuel from both my tanks, each with it's own valve.
This helps balance the aircraft with more in the right tank, one up.
I also switch tanks at certain gauge points to better estimate fuel quantity remaining.
For example, if I switch from the left tank at X on the gauge and then run the right tank down to X on its gauge, I can cross reference that (known amount) with the flow meter reading.
I feel happier being in control of my fuel than just sitting there "wondering" what's going on.
As for the return line, I feel high wings with a good flowing fuel sytstem don't need one. (No choke points to the pump under 8mm dia and good flowing filters.) There is already head pressure assisting flow and working against vapour lock.
I can understand why Rotax brought out the SB. To cover their asses and all bases.
A low wing (with no electric "push" pump) on a hot and high day is a recipe for vapour lock without a return line.
If people are happier fitting them that's ok......