I don't know about "Detrans", although anything that helps refute transgenderism is useful at least in the short term. However, I do not see TERFs or Gender Critical as an ally. At the end of the day they are feminists, even if they reject transgenderism they accept numerous other more damaging degeneracies.

Enemy of your enemy is not necessarily your friend, especially if the enemy's enemy is your enemy anyway. People should be careful who they ally with. It's similar case to how the early alt-right and the alt-lite tried to form ties with ex-Muslim communities to attack Islam, but ex-Muslims are usually a bunch of degenerate globalised hedonists pushing globohomo and most people finally accepted they were useless as "allies".