From an electronics point of view I'd make it work the same as the Novak smart-stop widgets, but triggered by time, not voltage.
That way it will work on any R/C electronics and doesn't have to switch high currents, you still get steering, but you have either no drive or very limited drive until you disconnect / reconnect the battery.
Could be done with just digital logic, but easier if it was a microcontroller as then you can choose things like with 30 seconds to go the epa drops to 30%, then cuts off completely.
Cost for someone to build that might not be cheap though, unless you have a mate who will do it for you. For a small electronics company it's probably at least a £500 job (1-2 days work) + parts + ~£50 cost per unit, maybe more.
Would probably be ~2 week turn around for cheap china PCB to be made too, depending on what you want the final product to look like, which limits how fast things can happen.
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