I have heard that the secret to Theraphosa producing sacks is feed them worms. Apparently in the wild the females eat nightcrawlers. I have not tried this yet, but I am going to, just haven't picked any up. I just bred another stirmi, so I need to get on it. I had one stirmi drop in may and I pulled it right away and manually incubated it, but when I opened the sack all the eggs were black. I was really disappointed. Good luck with your breeding and keep us posted on what happens
Interesting thought, I have some slings that I am raising, we will see what happens when they mature, hopefully I will get a fertile sack eventually. I have two females bred right now, but they are WC stock