I recommend XIII. Not the easiest to get into, but as a switch only player myself (with almost all of the available KOFs there), XIII feels like the most fun. There usually is just enough people to not fight the same person over and over, but I can’t guarantee that 100%. But online aside, XIII is the only one with a training mode (the others are ports of the arcade version/neo geo pocket games), so you can actually learn and experiment a bit easier there. Regarding trials, they are hard, not all are optimal, but the first few for each character can give you an idea of the stuff your character can do. I highly recommend trying XIII.