Impressive report lene. Dont be discouraged, I hope you can succeed with those bimac babies.
Just as open question for the forum, may be time for starting with small crabs instead of shrimps?
perhaps
Xhanthidae, those that occurs frequently on Acros and SPS and that nobody wants there. Other possibility would be small Mytrax,
Porcellanid.. perhaps Coral commensals like Tetralia or Trapezial species etc... To get them, here we have some LFS that "shower" the live rock and corals over a net to get them out, and do nothing with them.
On the other hand, last week I got a couple of Calcinus Spp from my LFS really tiny, just about 2 mm... I dont know if could be another food option? Nut has never been interested in hermits, but I've readed that small octos would be.
I'm completely new with briareus but seems 2me that crabs produce a higher hunt response than shrimps. Just brainstorming, hope someone more experienced may point the right direction.
Good luck! cheers