Do what djmath said but do it with every single one of your attacks. Don't worry about pivot anything until you can L cancel all of your attacks at will. L cancelling is VITAL. When you are practicing with said bowser, also practice spacing simultaneously by tippering all of your attacks. To be sure you're L cancelling you can do a move afterward a gauge how long it takes to come out. Use something like jab, dtilt, or grab and use the same one to be sure you're L canceling.
Practice Dash Dancing until you never ever under any circumstances get stuck in the slow turnaround animation. Like Espi said, work on your wavedashing and play with the lengths. Start on one edge of a stage and WD to the other edge. When you find out the max distance WD length, it will take fewer WD's to get across the stage. This can be helpful for movement in general and taking the ledge quickly.
Spend time practicing getting to the ledge quickly with FH bair FF, SH bair, and WD back FF. Then practice recovering from the ledge. Marth's ledge recovery options are limited so you need to get those down. Practice ledgehop fair, nair, uair, and even dair. And practice L CANCELLING those. Also WD onstage from the ledge. Marth's ledge invincibility is negligible so don't worry about WDing on invincibly.
Also keep bowser on a platform above you and lock his ass down with uair L cancelled and utilt. You can stop him from dropping through once you're fast enough.
Lvl 3 fox and falco are likely best to practice uthrow chaingrabs on since they always DI behind you, but know that real spacies will not react nearly the same way CPU's will.
Finally practice movement. Go look up PPMD's matches especially against M2K's evil sheik. Look at how he combined WD and DD for godlike movement. Try to emulate it by moving around stages alone. Do this with a 2nd controller plugged in, a lvl 1 CPU and constantly be out of his range, or the 1 player mode glitch. You can also play around with fixed camera mode for this.
You can practice the basics alone but things that actual people do are neigh impossible to practice. I'd suggest practicing everything with time set to 8 minutes as this allows you a small break and simulates an actual match going the distance. You will never in your melee career need to play balls to the wall for more than 8 minutes with no break.
Good luck bruh.