We have needed to use Miralax for my son, who is now seven. I have a very conservative, low-intervention pediatrician, an he said it's always better to treat any bowel problem from above (starting orally and moving through the digestive tract) than from below (with suppositories and the like). Miralax is inert and just adds moisture to dry stools; since our son has an extra-long colon that removes too much moisture from his stools, this was the right fit for him. We used Miralax for a week or so at a time with a dose every day, and then when we had a couple days of regular, non-straining stooling, we went back to a regular high-fiber diet or occasionally something like Senekot, which was also recommended by my pediatrician. You might like it, since it's all-natural--it's pretty much baby prune juice, concentrated. The Miralax was just to get re-regulated.
Human milk has a natural laxative effect and definitely qualifies as "natural"! If you are no longer nursing, consider asking a friend who is for some milk to offer your daughter in a sippy cup or the like. Maybe that weirds you out, but it would be very wholesome and nourishing while softening things up.
It also sounds like she just might not be ready for toileting.
You definitely do not want this to turn into encopresis, since that can damage the colon area. Getting things moving in a regular (and that may mean less than once a day for her) is important.
Best wishes!