Hi, i'm Danny, I hope you are well.
From memory, these use the abs sensors to calculate the speed from the wheels and this information is used for the speedometer, etc, as you have already said yourself.
When testing abs wheel sensors and other similar sensors, a oscilloscope is needed, not a multimeter, the signal is effectively a square wave signal, not a increasing or decreasing voltage.
P0500 is vehicle speed sensor fault, in the case transmission based speed sensors. In this case, it can be lack of communication to the abs module or lack of signal from the abs module, again, as you have already suspected yourself.
My concern is that the cost and effort to replace the abs module are reasonably high, with no actual proof as yet, I wonder if you are able to get to a local Autozone or similar and have the codes scanned, with a unit that is capable of reading all the modules in the car, i.e., the ecu, abs and dash, as opposed to a cheap obd reader that can only read basic ecu fault codes.
I would then suggest you report back with the information and dtc's obtained.