So far (just over 1000 kms) my X20 experience has been great! I ride my carbon gravel bike with 38 mm slicks everywhere now, most of the time with the system turned on so the MAHLE head-unit will display, but with 0 assist. I don't feel any drag. On the climbs where I would shift my gorgeous Italian steel bikes into the "granny gear" on Campagnolo triple chainset, instead I move the assist to level 1. It's still a workout but just a bit easier. I tried a few climbs at 0 assist but dragging the extra weight of the X20 system uphill for very long wore me out so whacking the button for the electronic "granny gear" is my preferred scheme. Once I hit the peak I go back to 0. I noticed the system seemed to crank up assist in level 1 if I stood up on the pedals, so maybe the torque sensor is doing something? I pay zero attention to cadence so can't help you there, same with inclination.Why not start with a conversion of your current road bike? You already and the fit right, add a 350w-500w mid drive system should put you where you want without breaking the bank even if you hire someone to do it for you. You didn't say where you are located so I have no recommendation for you as to where to look for help doing a conversion.
Meanwhile, my wife rides X35 bikes (gravel and road) using a heart rate monitor to control her assist levels. She checks post-ride data and finds she rides 1/2 the time with 0 assist but doesn't complain about feeling any extra drag over her gorgeous Italian steel machines, just that nice "tailwind" when her heart rate hits the preset max. So far, BRAVO to MAHLE!