Holding poop like that can be harmful to the bowels... I forget the name of it, sorry.
She is 5 years old... so she can understand "logic."
For us and our daughter, when we explain things logically and a topic that has to do with her body,, it works for us. For example: if this were my daughter, I would explain to her that her diarrhea was because she was sick... she is not sick now.. but the body still has to poop... and if she does, it will gradually get better. But is she does not poop, then it will get worse or her body will not "heal" and then other problems can occur. I would tell my daughter that a body has to poop, because it is the way the body works... then I would show her a model of the body (which we have just like in the Doctor's office) and I would show her how once she puts something in her mouth to eat.. .how the body "digests" and then into the intestines, and out it comes...and then you go to the toilet.
Or, we can go to the Doctor... and see what she says...
Or, I would go online and do a search, and SHOW my daughter what happens if she holds her poop and doesn't let it come out...
For my daughter, these types of "explanations" work... so that is what I would do, in her case.
Emphasize to your girl, that her body has to "heal" and get better... when I explain this to my daughter, it clicks with her.
You have tried many things, and appropriately so.
If it continues, I would go to the Doctor... you don't want complications to occur... or, you may want her anus looked at in case she has tears/redness, which causes great pain.
All the best,