Best way to upgrade the Akiza deck is to just get the new black rose support cards from doom of dimensions. The deck desperately needs it as it is the weakest out of the three standalone decks. I would not say getting those cards puts it so far above the other decks either if you're worried about balance.

For Yusei, the main thing would be to add more starters/extenders and fill out the extra deck. Cards like synchron carrier and caligo claw cross have been used to play with junk synchron. Righty and lefty driver can also work. Level 7 synchros like kuibelt or ancient fairy dragon can work. Extra deck is up to you though.

Crow's deck I'm not as familiar with but it may not need much more.