I have a few top scores in the 60 million point range, so I'm not the best but not the worst at this game either. I watched a lot of Karl's streams so most of my strategy is his but modified for my 'skill' level.
Ideally, you want to play all 4 multiballs at the same time, as the jackpots are multiplied depending on how many multiballs are running. I typically go for the lights multiball first because my priority is to lock balls, and get the patches that spot band members. Once I've started lights multiball, I go for pyro. To start crowd multiball its best to use two balls on the right flipper and backhand the crowd shot. Then I shoot ramps until I get amp multiball started.
My gameplan usually never works out irl hahha so I typically ignore the jackpots in all the multiballs and use them to go for patches and guitar locks. I go for Fender, gibson, vater, marshall, to get the extra ball. I usually end up playing a song before I get all those patches though. Play the song, and if you get a low song score, concentrate on completing all the inlanes so that when you collect the jackpot it lights extra ball.
Depending on how well I am doing the next set of patches that I will collect will be either the Tshirt and poster - to light extra ball, or if I am still on ball 1, I go for the rose, duff skull, franks drumsticks, and melissa's candy. Every song I play I concentrate on lighting extra ball by lighting the inlanes and I ABC everytime. Always Be Cashing if the extra ball is lit! This is the opposite of Karl's strategy which is to go big, or go home.
If somehow I have managed to collect all of those patches, I will then go for the multiball patches. But typically I have played 3 to 5 songs by then lol, and my game usually ends before I can get any further.
I feel that the more songs you play the higher your song score will be even if you do poorly in the multiballs. Choosing a song does lower the song score as well, so I don't choose songs, unless it's coma - which I think is the hardest one to score well in.
I always disable album modes when going for high scores, and I always time out slash's solo as well.
Concentrating on getting extra balls is kinda my way of getting high scores. I had one game where I played 8 balls lol! 1/3 of my games are over 15 min long.
When I get bored of my strategy, I'll probably try to play the boosters the way they were meant to be played and concentrate on getting jackpots. I don't see any reason to change it though until they update the code to balance scoring, and finish a few other modes in the game.