Oh no, sorry to hear that.
Unfortunately that does happen sometimes
I didn't like platys to begin with because they are a lot plainer than guppys but the male I found was a hi fin and I hadn't seen any like him. The female is also a hi fin but also a mickey mouse platy. The other female platy was in a tank of swordtails. She is the same red colour as the male but has 2 black bars on the top and bottom of her tail.Tbh I gave up with guppies, like you, no matter how good I kept the water, they died, though the babies that some had survived & did well.
I have platys now, with some Oto's & a female Betta
Had no problems at all with them, in fact they're doing so well, the males are going to the Lfs this week
.... I will not recommend a breeding trap for the birth process, but you may want to move your fry to a breeding trap (for a while) after they are born..... Your setup is reasonable for a portion of the fry to survive...
do not fret!!!.... the more you check them out and fret about them.... the more you're going to loose.... It is just like that.... So let nature take it's course.... you are set up... and the fish will take care of the rest....