I don't think it needed to be completely mapped out. There was a "plan", and JJ has made that clear, but like the OT it was very nebulous and changed when big answers were changed. To me the issue was simply hiring Johnson.
I think the problem was giving Johnson too much creative control, not Johnson himself. Rian Johnson is a good film maker. Go watch Knives Out and you will see that. You needed the presence of a producer who would tell him "no", and there was no one there to do that. Everyone got pissed at Feige for interfering with directors like Edgar Wright, and Alan Taylor, well the simple fact is it wasn't their story to tell, and Star Wars was not Rian Johnson's to tell. There were many good things in TLJ, they are just overshadowed by the enormous amount of bad things that make it a train wreck. It started with Luke. The decision to make him a guy that quit on everything, wasn't a character arc it's a shot out of left field, and betrays who the character is.
A movie isn't made by just one person. Even a film that's a singular vision like say Clockwork Orange, it's the combination of actor, director, writer, producer, each doing their part to make a product.
The Empire Strikes Back wouldn't be the classic film that it was if it was only George Lucas. The combination of Lucas with Kasdan, with Irvin Kirshner and Kurtz providing input, with having a strong cast of actors to work with. It's the combination of those things that brought that film to light.
Look at the difference between those films and the prequels where you have primarily Lucas making what he wanted, with "yes man" Rick McCallum basically allowing George to do whatever he wanted with no constructive input.
Kathleen Kennedy is someone that it my opinion is a good producer when partnered with the right people, like her work with Spielberg on Raiders of the Lost Ark and E.T., but she is not someone fit to be the sole creative voice pushing things forward. That's just not her strengths. George trusted her to run Lucas film because of their past relationship, but just like a guy who is good as an offensive coordinator, gets promoted to being a head coach and then they are terrible, the aspect of being the head of a studio such as Lucasfilm is not suited to her strengths.
This is blatantly apparent when both J.J.'s and Kennedy's test screenings miserably bombed, and they had to bring in Lucas. And that brings up another point, yes they did have test screenings, every film does, I don't know why in the hell J.J. is lying about that in the media.