The reason for the wood in the bottom is to store moisture, not to produce nutrients for the plants.

So you should prefer already weakened/semi decomposed wood (just pick old tree branches off the ground in a forest).

Besides, as the linked article explains, a typical mound has a lifespan of 5-6 years [1].

I don't see how your argument about human lifespan is relevant here?

1: https://en.wikipedia.org/wiki/H%C3%BCgelkultur#Lifespan