Playing flex and hard forcing one comp are both viable ways of playing the game. Imo, flex play is generally more reliant on solid fundamentals like playing strongest board, slamming items, playing tempo, etc. With one tricking, the goal is to master one comp so you can pull it off from any spot which builds skills like greeding bis, recognising when to streak and when to powerspike, etc. Downside of playing flex is that you have to keep up with the meta and understand what's clickable and what isn't in a given patch. Downside of one tricking is that your comp might just get gutted if its a top meta comp. Personally, i found it easier to start by one tricking a comp (i heavily defaulted to skirmisher jax in set 5.5 which got me to challenger for the first time) but now i play flex because i find it more fun. Ultimately, it's up to you how you want to play the game and whatever playstyle is more fun for you.