Here's what worked for my sister when she was potty training her children, who had similar issues during the beginning of potty training.
She allowed her daughter to wear a pullup to poop in, but the rule was that she had to poop in the pullup while in the bathroom, preferrably while sitting on the potty. Once all pooping was consistently happening in a pullup while sitting on the potty, she cut the bottom out of the pullup and made a pullup "belt", which allowed the poop to go directly into the potty, but her daughter still had the security of having the pullup one. Once my niece saw that pooping in the potty didn't hurt or cause her to fall in or caused her to be "flushed away"(her underlying fears of why she refused to do it), there was no need for the pullup "belt" anymore, and she was potty trained.
I think that there is something about the effort a poop can take vs a pee, that makes smart little ones fear for their safety with pooping on the potty...