You are not being naive. You've merely rediscovered one of the most regrettable issues in all of aviation, and you've put your finger squarely on it. The relationships between prop design, blade count, diameter, tip speed, torque, and horsepower for propeller -driven aircraft are completely goofed up. We're stuck in a hysteresis where our engines come largely from our torque delivery requirements, which come from our props, which are designed (poorly) for existing violent, ancient engines, so the question of what to prioritize depends on what you have under control. Whole systems thinking is a 'luxury' the industry has not supported.
Really, none of the status quo is anywhere near where it could be. Your thinking is correct and headed in the right direction.
The observation you have joined many brilliant folks in making is glaring regardless of whether one looks at it from an engine perspective, an aerodynamic perspective, or from acoustic, structural, thermodynamic, or mechanical points of view.
Why is such a correctable cancer tolerated? A weak answer is that our aircraft engine opportunities have been rigidly slaved to issues of certification, even within the experimental segment, due to indirect coupling. The costs of certification for the installed base dictate our choices and our 'known dataset'. But the uglier truth is that we don't collectively know what to target yet.
I hope to help make some of the solutions obvious soon, when those of us on the same page unite to overpower ignorance. It starts with re-examining the aerodynamic issue from top to bottom, where from quiet and efficient props we will learn that the powerplant issue is torque at low RPM: we want far more of it.
Due to their relative torque independence, electric motors are the perfect catalyst for establishing and overturning a huge number of myths associated with aircraft propeller design; myths that depend entirely upon the perpetual reign of ancient air-cooled aircraft engines. We are not alone, and the vanguard will be led by those paying attention to what guys like Jack Norris, Paul Lipps, Howard Handelman, the rest of the PADA, and the CAFE Foundation have to say.
You make one of the most important points that needs to be made when safety is priority one: attaching an aircraft propeller to an automobile crankshaft is a DUMB idea, done all the time. The gyroscopic forces acting on an aircraft prop are INSANE. One of these days I'd like to rig up a 'kinetic display' of gyroscopic prop forces for AirVenture visitors to wrestle with. It would have an equivalent angular momentum, a disk spinning at 2700 RPM, hooked to a non-rotating crankshaft for its handle, and visitors could finally see and understand what they are asking their auto-conversions to do. Bent crankshaft, anyone?
This finally brings us to the PSRU (Prop Speed Reduction Unit, prevailing jargon for gearbox).
What a can of worms. I'd rather take back everything I just said about bolting a prop onto an auto engine and stick a fat, efficient little thirteen blade unducted fan on the back of a direct-drive V-8 pusher than wade into the cold, deep waters of PSRU design unarmed.
As others have said (well), it gets complicated. Fast. Harmonic resonance issues and astonishing power pulses in both driven and driving directions are far too often not considered adequately even when the biggie (gyroscopic forces) is tamed, which is rarely. Lightweight solutions are costly by nature, and the market balks. Therefore the problem is that we tend to adopt either of two dumb ideas: no PSRU, or PSRU on a budget. There are better ideas coming.
The beauty of this organization is that it gets enough smart people frustrated enough to collectively fix things. Thank you for calling it as you see it.