You seem to equate souls to anima while, to my understanding, they are not the same. Souls are not comprised of anima but merely contain it. A soul without anima is still a soul, it doesn't need it to function (with the exception of Wild God/Ancient/loa souls for whatever reason). If that was not the case, all the souls ending up in other realms of the Shadowlands would cease to exist the moment their anima was taken away which doesn't happen.
As for souls assuming animal forms or simply becoming tenders being few and far between, what we see in-game does not necessarily represent the actual scale of Ardenweald or its population. For all we know, the Ardenweald forests may be teeming with these souls but they are not shown to us for gameplay reasons.