Can't get enough braking power after lots of adjustment
Render R's are very susceptible to the arm being over-actuated. They don't hit a physical stop like some of their peers, but they reach a point where the cable anchor location is hanging in space and can't be pulled anymore. That you can't get good power even braking from the drops suggests this is the problem.
Detach the cable. Set the barrel adjuster all the way in. Loosen the caliper mounting bolts. Anchor the cable with the smallest amount of arm actuation you can while avoiding slack. Dial in the stationary pad adjuster until the lever feel is what you're looking for as a final adjustment. Tighten the caliper mounting bolts and get your pad gap dialed by squinting at it good. Even if getting free of rub is tough, trying this may tell you why power was lacking.
Like all mechanical disc calipers, this brake benefits highly from compressionless housing. Faced posts and compressionless usually let this brake perform fairly well. Also like all mechanical discs, small issues with the cable and housing setup can result in major issues with power and the levers bottoming too easily. Badly prepped housing ends, or housing ends not fully seated in the levers, can be an example.