Stiffness is not usually a feature of cable steering. Check another boat or try a new cable.
Cable and pulley steering done properly is the lowest effort steering I know of except for power steering. But more expensive that either push pull cable or hydraulic.
You don't say whether you will do the work or not....
Hydraulic systems are not complicated as most mail order places and/or manufacturers have tables that you align your boat specs to and they have complete packages for your specific needs or even boat.
You may also explore for a classic Bertram owners site and ask there...I'll bet there a dozen possibilities offered in moments.