Well, completely separated batteries you switch between with a toggle (or whatever) are "simpler" than figuring out the batteries or dealing with plugging / unplugging for charge / discharge stuff is probably closer to "dunce proof".
If the "battery combiners" out there were correctly documented for what they actually are internally, instead of so many made-up things to make them sound better for marketing, I'd recommend using one of those. A simple "ideal diode" setup would work, and some of them probably are that. But some are supposedly "smart" and without knowing the actual internals and the actual specific logic used, behavior can't be predicted and thus safety can't be guaranteed.
So...if you use a simple DP3T (2P3T, On-Off-On) toggle switch rated for *at minimum* the max possible current your system will ever draw under worst case conditions, then it's pretty safe and foolproof. Sometimes these are called DP2T or 2P2T center-off instead, since the center throw has no contacts some manufacturers dont' count it, which makes it confusing.
You use a double pole so that you can switch *both* negative and positive wires, completely isolating the battery not being used at the moment.
You use a triple throw so it has three positions, using center as off. That way no possible normal failure of the switch can cause both batteries to be connected together at any time.
The common contact of each pole of the switch goes to your controller / system input, one for + and one for -.
Then the corresponding poles for each outside throw go to the + and - of each battery. If the switch has poles for the center throw, leave them unconnected.
If the speed sensing is being done via something other than a motor hall sensor, then under some conditions the PR can swtich to reading the hall sensor instead, and that will screw up the speed and mileage readings because the hall will have many times more pulses per rotation than the typical wheelspeed sensor, as the motor hall will be reading at least a few dozen more poles / magnets.
