Sorry to hear you have terrible times trying to keep livebearers. They are the main focus of all of my tanks and I had this girl for about a year before she dropped the fry in the picture. BTW the fry in that picture also command a premium price at club auctions and are now as big as their mother was in the picture.
I also have a wide assortment of endlers and AOC guppies. My swordtails are now a year and a half in my tank and the X helleri are finally looking really nice. My platies do not do as well as my swordtails but they are highly diversely bred, not at all inbred, and not as well selected for health as most of my fish are. Most often I end up with some platies on a whim rather than selecting wild type stock. If you have a healthy water system with relatively hard water and do the needed upkeep, livebearers seem to just go on and on with almost no effort. I have seen them repeatedly mentioned as "just add water" fish. If they are fed and the water is well cared for they are as tough as nails.
My AOC guppies were bought from a typical fish shop and looked OK but not really good enough to put with my other fish. I was a bit concerned about them not being entirely healthy. After a few months the "highly inbred and weak" fish came around and now I sell them and their progeny at a premium to other fish keepers who just want quality fish. My tiny colony that started as about 6 adults now numbers in the dozens after I have sold off about 30 of them. Needless to say, mine are highly inbred because I just remove adults now and then for sale and let the healthy fry grow up to breed. I do not hesitate to remove any fry that are mis-shaped because I understand that cleansing the gene pool requires it. A highly inbred population that is culled regularly produces a superior fish, not a weak one. The problem of inbreeding is well known because of the problems seen by some of the european royalty, but they were not culling defectives. Any good breeder of almost any kind of animal will tell you the benefits of inbreeding and regular culling for improving a line. Failing to cull ruthlessly is a crime when you are trying to improve a line of any animal. It can cause the line to degenerate to a lower quality level rather than improving.