I’ve read this topic with interest, the problem with the pump failing and superheating the standing water in the unit can be easily solved with the type of fitting used to dump water on carpet cleaners truckmounts, it opens and closed at a set temp, put it on the exit pipe and this will protect the unit
Or you can buy a blow fitting that is used to protect pumps against incoming hot water on some pressure washers , this is a small plug that blows out at a set temp although it would cause a water leak once it activates it would also do the same job