Am I absolutely wrong or is it not actually much of a factor?
On the other side, large planes carry a lot more fuel as a percentage of the total mass, it really matters to them.
It shouldn't be a large factor. Drag is independent of mass, and while lifting the fuel to cruising altitude takes energy, most of the flight should ideally be cruising.
The rocket equation doesn't apply; that's for an entirely different regime.
Also, bear in mind that it's much easier to accurately and reliably measure a battery's state of charge than it is to measure how many gallons of fuel are in an airplane's wing.
I don't, however, get the point about propeller efficiency, as is it not about matching the pitch to the airspeed? Fixed-pitch propellers have a maximum efficiency at one particular airspeed (and maximum thrust at, in general, a different particular airspeed), regardless of what sort of motor is turning them.
If you play with the propeller pitch, the force on the engine will make it change the rpm; if you change the engine rpm to the optimal of your propeller, you don't always have the best pitch, so you adjust it and this will impact the rpm. With an electrical motor the power delivery is much more robust, so you can adjust the pitch and then set the motor to the desired rpm and that's it.
Normal operation of a constant-speed propeller involves setting the desired rpm via the pitch control, and then adjusting the power, via the throttle, according to your purpose (such as level cruise at a specific IAS, or climb at maximum sustainable power), with the propeller govenor maintaining the chosen rpm via pitch changes, and I do not see how electric power changes this relationship. I understand that electric power might simplify making a sort of FADEC for electric airplanes (so that there is only one power control), and that constant-speed propellers could eke out more duration from the batteries, but now you are adding the cost and complexity of a constant-speed prop. I am not sure that it would be justified in a light airplane having a cruise speed not much higher than its best climb speed.
There are many planes in Europe that have the cruise speed much higher than the best climb speed. The best climb speed is around 100-120 km/h for light planes, while several plane models have the cruise speed over 200 km/h, up to 250 km/h. I am not aware of any plane with cruise speed below 200 km/h to use constant speed propellers, they have fixed pitch propellers, but the faster ones have in-flight adjustable pitch. I personally flew 7-8 different models of planes, only one with adjustable pitch, but they were all in the LSA class and const-speed is not used there.
Later edit: I did some calculations, the result is less than 30 minutes.
Another positive impact to GA is you don't need to adjust for mass and balance that much, batteries have a fixed center of gravity that does not change while discharging, while fuel tanks impact the plane's center of gravity as they go from full to empty. Less trimming, simpler flying.
And not the last, it is eliminating any problems with carburetors, mixture, air density or inverted flight: none of these affect an electrical motor, but it needs to be considered for the likes of Cessna where you need to adjust the (fuel/air) mixture depending on the altitude.
And it improves propeller efficiency, simplifying constant speed propeller operation. It can be easily fully automated.
All good news, but seeing it happen is a different story: it can take 3 to 5 years, more that a decade or never to reach mass availability and adoption in general aviation. It has the potential to make the market explode (in the positive way), but there is so much politics involved there are chances the status quo will be maintained. Let's see.