Just like Sakura said do a few more rounds of the treatments. Especially if you think it helped, meaning that it working for the problem they have, then of course do a few more rounds. You already know the symptoms for internal parasites- white , clear, broken white stringy wormy shape poop, bloating, twitching, discolored stomach. External parasites symptoms- scratching,darting, white salt grains on the body. Also gasping for air symptoms for the gill flukes .
So weird that all bettas got sick, especially that you very careful with not sharing anything.
I saw the pictures it does look like biting. Divided tank you can try , i don't like divided tanks because if one betta get sick , you ending up with 2 sick bettas. I just read 2 threads like that. Sorry for being negative it just my personal opinon.