I guess the goal is to start the game with a group of players with detailed backstories, character connections with each other, lots of deep lore, connections to the world, immersed in the world, and other things?
I'm not sure the "we must play through the prequel, before we can start the game" is a good way. Most players would want to start the game at the "start" of the big action. And even if you take five hours to do five paths for five characters with a "the story will start next week", it might not work out. Adding the 'prequel' will just delay the game, so many players will want to rush through it. And if you rush, then it's not worth it. A PC meets a couple NPCs and such at a fast speed, then on to the 'story plot'.
Worse, even if you get to a good spot in the 'prequel', you have the Prequel Problem. If you have Han and Darth Vader fight in a prequel, then you have to have awkward things like where Vader is like one second from killing Han and suddenly for no reason he lets him go. Because 'Star Wars' must happen....
I think the answer is just to straight up play the game. Right at the start of the adventure. Just do all the backstory and other stuff during the game. Think of levels 1-3 like the 'prequel' times.
Eventually, if your playing with a good group, you will get to the sweet spot.......then game on from there...