Can't get enough braking power after lots of adjustment Hi :) I just bought a 2015 Specialized Diverge A1 with almost completely stock components. With some adjustments everything seems to work well except for the disc brakes.
This bike has a Shimano Claris 2x8 groupset and the mechanical disc brakes are Promax Render R.
I've ridden other road and gravel bikes before with mechanical disc brakes, and never felt unsure about stopping power, whether from the hoods or drops, until now. That's just to say I know what a brake should feel like, and I have decent hand strength. With this particular bike and brakes, I can hardly squeeze the levers if I'm riding on the hoods, and even if I get maximum leverage riding in the drops and can squeeze all the way, the stopping power is terrible. There's no hope of locking the wheels even just going 5mph. It's a very slow, weak rolling stop.
I've tried checking and adjusting just about everything I could think of or google, to no avail. I'll go through what I've tried:

realigning brakes

barrel adjusters

adjusting the internal pad to rotor distance (don't know if there's a better term for that) both closer to the rotor, to see if less travel and squeezing as hard as I can would help, and farther from the rotor, to see if more travel made it easier to squeeze the lever from the hood. Closer to the rotor was slightly better.

inspecting and replacing pads. They didn't look very worn or warped (definitely at least 3mm of material left on each pad) but i replaced them anyway

rotors do not look warped/bent either

cable tension. Can't find a happy medium where there's enough tension for the pads to clamp hard on the rotor and there's enough slack that i can move the lever effectively from the hoods.

cable/housing condition. Looks very clean. If I release all tension from the cables, the levers move freely. I don't feel any friction.

these levers don't have the built in reach adjustment, so I put in shims to bring them a bit closer to my fingers to make it easier to squeeze. Not enough help.


In summary,
I can't get enough stopping power with my disc brakes, and I'm also finding it practically impossible to squeeze the levers effectively from the hoods. They feel so stiff.
In terms of my inability to squeeze from the hoods, do my hands just hate this Claris lever/hood geometry? Are they weaker than I think? For the lack of braking power, is there some adjustment I missed? Is something else going on?
I'm doing all I can to fix this myself even though I'm a total novice, so any help is greatly appreciated :)
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.