If you can get a Turkish or Manchu ring working properly, it should take way less strength to hold the string? Ie the ring gets pushed to the sides of the thumb, if I understand correctly. I haven't, though. Manchu archers pulled ridiculous draw weights, sometimes a little over 200lb (very rare, upper limit. average was more like 120lb or something, similar to English archers).
It's all about practice. The more you use thumb draw, the more your thumb will gain strength etc. I can draw mediterranean bare fingers more easily because of the greater surface area but with a thumb tab or ring I can draw about the same.