Joanie, I highly recommend the Ikea potty. It's the one in the picture I posted and it's only $4. Get three or four of them and put them in convenient places in your house, in your car, etc.
So, Sora is straining to poop for hours? That hasn't been the case for any of my babies. It kind of sounds like constipation! Usually breastfed babies do not get constipated, but I do know that it's possible. The other possibility is an allergy. You mentioned that you're eliminating things from your diet? How long has that been going on? Is Sora on probiotics? Does she poop daily or go days between poops? Generally babies stop freaking out about pooping by the 2 month mark, plus or minus. So around now. My babies seem to out grow their elimination issues by the one month mark, but every baby is different.
My experience with poop has been some grunting, a "poop" face, pushing while flexing limbs, etc. and then the poop comes. Usually all at once, but sometimes in two sessions separated by a few minutes. It's always been my experience that if I get them on a potty in time, all the poop will come out at once. I would suggest trying a different position with Sora so that it's more comfortable for both of you. The root of elimination is the ability to relax and let go. If she senses your tension and can't relax then it could be harder for her. I prefer the position I'm using in the pic- everyone seems comfortable.
Nice job catching those pees! It feels good doesn't it?