Hello techAyan
Well, you can start making your game after the jam announced the theme
Also, i think the better practice that involved making game (as you said) is to join other game jam and enjoy your participation , There’s many jam that has started or about to start and GD game jam only had 1 month left before the jam announced.
So take your time to practice yourself and don’t forget to participate this jams 🫙. (I mean GDevelop game jam).