Maybe look at the Tektro RL340? I haven't used them but they look like will do what you need. Perhaps someone who has tried them can provide some feedback? I have TRP RRL levers on one bike, but they don't have a return spring. In retrospect this may be better.
https://www.amazon.com/Tektro-RL340-.../dp/B003U9PYZY
I'm sure there are other candidates as well. All modern levers are aero. If you use silver ones, they won't look too out of place on a vintage bike.
Campy Record brakes were famous for their stiff strong springs. It was seen as a mark of quality at the time. You
might be able to find a compatible weaker spring from one of the many clones, but I would recommend against that. A modern lever with a counter-spring will lessen the apparent force more effectively anyway.
CORRECTION: After thinking about this, I realize it was the total design of a weak spring in the caliper and a weak spring in the lever that helped reduce total spring force.
So it's a combination of a stiff spring, lack of leverage in the single pivot design and no return spring in the brake lever?
More or less, yeah. I'd say different leverage ratio rather than lack of leverage myself.
Older brakes did use a different leverage ratio, usually referred to as mechanical advantage or MA. The calipers moved more for the same amount of brake lever movement. They took more handforce, but the calipers could be set more open too, so you could get home with a broken spoke or whatever.
Also check your brakepads. Old dried out pads won't stop well and should be replaced.