How long has Tridax been collecting Takanuvas for his collection? Was it within the few months after the prime Takua put on his mask/fought Makuta? Or has he been collecting them much longer, and Takua’s true identity was already well known within the Brotherhood?
In Entry 5 of Makuta’s diary from Makuta’s Guide to the Universe, he appears to have not known that Takua would be the one turned into a Toa of Light by the mask:
Takua himself turned out to be the destined Toa of Light, Takanuva.
As this is essentially a private record, there shouldn’t be any reason to assume he’s lying here.
In that case, Tridax must have been pretty busy within the following weeks, parsing through the alternate universes for the Takadoxs to have built a even a small army.