Which is why I said: “…as long as the device you connect to the 14ga wire will never draw more than 15Amps”, which would specifically negate a 20A outlet. That would be a code violation.
If the end device (the toaster or heater or whatever) has a fault the breaker/fuse in the fusebox should cut everything off. End of story.
Just for a bit of diversity, in Ireland/UK the method of over-current protection is done differently.
A breaker for a socket circuit will commonly be rated at 20A @ 240v, but a device on that circuit will be limited to 13A when using a “BS1363” plug and socket. The device will be fitted with a cable which is over-rated to the plate capacity of the specific device - for example: a 250W mixer will have a 1.5mm^2 cable, but we’ll protect that cable from the potential 20A on that circuit by installing a 3A fuse within the plug header.
Similar applies to a Shelly device - the Shelly should be powered off a plug and associated fuse which is suitably rated, or supplied off a “fused spur” (fused, double-pole isolating switch) if a plug is not wanted.