I agree that the brakes themselves are pretty lousy. I had a city bike that came with a similar (though Saccon brand Italian) cantilever on the front along with a plastic lever and all the effort I put into improving the braking was wasted. I would pull the lever until the pads contacted the rim and pull the lever 2" to the bar with no increase in braking force.
I eventually replaced them with the cheapest new V-brakes from Shimano (Acera, as I recall) that I could find together with off-brand cheap levers. Modulation wasn't great, but at least I could stop and put myself over the bars. It was cheap for me to do because I only had a front caliper.
Are you sure that the levers are short-pull (high mechanical advantage) meant for side-pull/centerpull/cantilever brakes? They're quite modern, but I can't make enough out of the picture to be able to search for them. Some levers are adjustable for either application.
I'd also disconnect the brakes and manually pull the cable through each length of housing, back and forth, to see if there's drag anywhere. I recently snapped a barrel adjuster during a move and the brake would periodically not open up upon release, and eventually the cable frayed from dragging on the jagged steel of the tilted fractured adjuster. I had to troubleshoot to find the root cause and replace the adjuster and the cable but reused the housing. Now she brakes just fine...