I tried 6400 series brakes with some slotted Superbe non-aero levers.

The 6400 brakes have that SLR feature- the brakes don't really have a return spring to pull the cable back- the levers have a spring in them to return them to the "ready" position.

If you use "old fashioned" levers, you have to manually "help" the lever back to the "ready" position.

If you're looking for non-aero levers and more modern-y Shimano brakes- I know there's options in the Dura Ace 7400 and Ultegra 600 6400 series levers.

Check out on eBay- they pop up from time to time- When I got my 6400 aero levers, I had the choice between the non-aero and aero- In the absence of the slotted levers, I wanted to be all cool and modern with aero levers.