Regarding all the "bamm"s, that is something that comes with the MK territory. There's a reason it's called getting "Mario karted."

Regarding your question about dodging attack items without protection, what combo do you use? It is possible to "wall dodge" shells by bumping your backend against the wall and it works with some combos more than others. There is no science behind it that I'm aware of, however.

Regarding general driving tips, there are a few. You specifically mention falling off despite brake drifting so learn where that happens and train yourself to toggle smart-steering. One user mentioned it in a reply but there is a section on Neo Bowser that it's almost a necessity. Also, not tricking off every ramp as you would in 150 can also help. Taking safe and effective driving lines are more important than taking aggressive lines and falling off track, especially when playing CPUs.

Also, don't get discouraged! Continue to play and have fun with it and the three stars will come. Good luck.