It took me 3 years to find a named Derketo priestess (finally got one last chapter). And most of my bases are built in the jungle, so I visit the Pagoda fairly regularly.
Just build a small base with a Transportary circle there and come to farm Thralls often enough. I’ve got a T4 Tempersmith on Official Siptah server in a week or two: just teleported there, killed them, rinse, repeat. If spawn rate for T4 Thrall is 1% then you have a 86% chance to get one after killing 200 of them (it’s a Bernulli rate).
Spawn rates are pretty low, but it’s better since AoS included a guaranteed priest spawn. Just keep trying. I’ve gotten a named male (accidentally killed him), female (about a month or so later), and a seen a few unnamed arch priests).
It’s not easy to get a T3 or T4 priest for Derketo. Regular priests and High Priests spawn quite regularly though. At Pagoda I’d Boindless Lusts just keep farming through all the camps and the Pagoda itself and one (T3 or4) will eventually pop up. It’s all you can do other than win the RNG lottery to get a very specific purge and then another lottery during that purge to have a high tiered priest spawn. You’ll have better luck to just keep running the Lemurian camps and the Pagoda.
The Lemurians being the tiniest of factions with only one location that has thrallable spawns is very rough. Even with the increase a year or so ago, they still total, what, 18 in the world at one time? With less than half who can be priests.
Purges could be an option, if you could get a Lemurian purge.
This one still maintains that Mitra (and probably Set) priests should be removed from the Black hand faction and replaced with Derketo and Zath.