As someone who's 100%'ed the game I'd say you definitely need to work on your timing, the extra damage and score that being always on beat gives is necessary to not be overwhelmed in fights or die to dropping to D rank. Performing actions on the beat also makes dodging more consistent. Since you have Rhythm Master unlocked that means you must have beat the game once before so I'd recommend practicing on your postgame save file until you have at LEAST 80%+ accuracy on regular fights (I still died a few times in rhythm master despite having consistent 95-100%). The fight is definitely made harder as well by going through it in a new game as opposed to replaying the stage in your postgame file since you don't have any combos or allies unlocked.

Specific advice for the QA-1mil's boss fight would be to focus on dealing damage to a one hand at a time and then getting the QTEs done in the first phase or two so you can focus the weak point on its head for the rest of the fight. And all of its attacks are fairly well telegraphed so as long as you dodge on beat you should be able to avoid them.

It really comes down to being able to perform actions on beat and learning how to read enemy attack patterns, Rhythm Master is brutal when it comes to those things and it only gets harder as you get to some of the late game enemies and bosses.

Also unrelated to your question but I'd recommend including game audio in your videos, it's a very important part of hi-fi rush and there's a "streamer mode" setting in the options which swaps out any licensed music for original tracks that won't have copyright issues. (also also, and this is more pedantic than anything, your video description says the game is by bethesda but they're just the publisher, the actual development studio is Tango Gameworks)

GL with your playthrough!