The problem is not to do with the interrupter levers (they are probably the least weak part), the problem is systematic.
Shimano and Campagnolo are known for their bike components, however, they can be seen as companies specializing in cold forged aluminium alloy components and able to churn out the best quality stuff, whether that be fishing reels, wheels for Porsche cars or bike bits. These companies have invested in really big presses for the cold forging process, invested in the finest alloys and invested in the really expensive dies that only make sense to invest in if the sales volume is there.
Only engineers inside the machine tool/cold-forging business or in the cold-forging business can really illuminate to you the science going on at the high end and what the likes of 'Tektro' miss out on. However, 'Tektro' parts are nearer being 'stamped out of cheese' than cold-forged properly. It is unlikely that their parts get put in the press multiple times, this forging process required to give the 'Shimano grade' finish, aligned aluminium alloy grains and consequent strength.
The Tektro brakes are well known for being 'spongy' and, as of yet, no Tour de France teams upgraded their Campagnolo/SRAM/Shimano brakes to 'Tektro' for that special 'spongy' feel. They are OEM parts without much call for them in the aftermarket. The processes used to make all of their parts are cheaper than those used by Shimano/Campagnolo. Hence the problem is systematic.
Given where you are, consider getting a better front brake, e.g. Shimano 105 (or even Sora). Also consider investing in Shimano cables. The back brake is less crucial as you skid on that anyway, but you might want to be on the lookout for a complete brake setup replacement minus the interrupter levers - keep them and swap out the rest as time and effort allows.