I have followed a couple of Youtube off gridders building their own homes. Their method for preserving lumber exposed to the weather is to use flame to lightly char the surface (both sides) of the boards. If your stump has lasted five years, now that you have charred the surface, you should get as many years out of the stump as I have been 39. In a couple of months, I will have been 39 for 51 years.
Tinker