I think the main reason Jade gets sidelined so much in Act 6 and post-canon is that she sufferers a bad case of the Superman Problem.
Superman: super strength, super speed, can fly, can see through walls, impervious to bullets, smarter than a human... it's hard to think of a situation he can't handle almost effortlessly. This is not good for storytelling! That's why DC ad to come up with the various flavours of Kryptonite and a roster of evil Kryptonians for him to fight.
Jade being a God-Tier Witch of Space is already impressive enough, but she's also 1/4 First Guardian, sharing some of the powers of a being that's supposed to protect entire worlds. On gaining her powers, she casually shrinks 5 planets down to the size of basketballs. Like you do.
But after that, whenever there is Action happening, Jade is out of action. She's knocked out and / or possessed (very frequent). She can't bear to fight something that looks like her doggo. She is sat in a wheelchair looking kinda out of it for unexplained reasons. I suspect the only reason she got to fight in "Bre8k" is that really letting her powers rip on the battlefield would have squished her allies too.
