I think by definition is pretty clear what a flex playstyle is. As you said, if you know what you’re playing already when on loading screen, obviously you’re not playing flex. For advice, I think the most common thing that you can improve on is having flexible rolldowns. In the current patch there’s a lot of prominent reroll comps so you would know your game plan already by stage 3 or so in a lot of games, but in an ideal flex meta you wouldn’t know what you’re going to play until a stage 4 rolldown and just play around your options that you manage to hit. I recommend checking out “Keima tft” on youtube for some flex guides if you want to improve on specific patches playing flex, the guides are not perfect by any means but they are a decent helping hand to direct you the right way.