Richard,

I need to understand your thinking. How do you perceive a relay will protect the motor and/or electronics? A relay is effectively a switch, so my question is really what quality is actuating the switch? Are you talking thermal overload? (a thermistor somewhere in the speed controller?) Some form of limit control? (already accommodated with the motor speed controller)? Sorry, I don't understand where you're coming from for "protection".

Similarly the timer module that you have there. From the outset I'd argue a couple of points - that module is likely (from a glance at the image of the board) to fry with the current draw from your motor (and I'm only talking of the principal motor in this thread). And in terms of "protection" you're only looking to introduce a timed on-state with that, and the only protection this effectively offers is against prolonged use (the quality here being the probability of over-temperature of the power devices in the speed controller). That presupposes that particular failure mechanism.

For the new controller and motor... tread with caution. If you use a long supply wire from the battery to the controller (increasing the resistance) you should introduce a level of protection that you can use to test, and then start to reduce in length as you gain confidence that the controller isn't overheating etc, with the motor under load. Ultimately you do want that cable length to introduce as low an electrical resistance as possible to allow you to attain the power through the motor that you expect
This is what I recommended to avoid overheating. It's a heath-robinson solution but it's robust.

There is another potential failure mechanism of back-EMF from the PWM output to the motor frying the electronics (if poorly designed - pure speculation on my part) - which
Have a read here... https://medium.com/jungletronics/dc-...f-589d8ed174cc, you might consider this a pragmatic approach
was intended to address. Here, add one or three capacitors across the motor terminals, or (I prefer, but seems to have limited traction in a number of articles) a TVS to suppress the back EMF.