The side decking functionality in best of 3 games works really well. I Think that same functionality should be added as an option to deck manager (like test hands is now).
Side decking is so important in modern yugioh, especially with the rise of multi-engine decks topping the meta. It would be helpful in deckbuilding to be able to practice swapping in and out cards to finalize ratios. It’s also just good practice (what are my side patterns if: I’m going 1st/2nd, what if I face X Deck, etc.)
As for implementation, on the deck manager screen there could be a button that says practice siding right next to test hands. This would bring up the exact siding screen that is used on best of 3 matches with the ability to sort and reset.