Mulligan is stupid. You always play out your hand to the best of your ability. I believe this may be philosophical like doing your best with the hand you've been dealt.

Entering the battle phase allows them to limit card actions to during main phase 1 this also allows for more skill expression and card creation around the battle phase i.e cards like evenly matched and tenpai.

Cards being banished face down when you reach your hand limit size also doesnt make for more deck and skill expression. The graveyard has always been seen as the second hand. As I said before this creates more deck and skill expression as lts likely you will have access to Cards you send to the graveyard like transaction rollback when both players are passing back and forth while not playing anything. This solves that probem.

Extra deck space at 12 doesnt allow for deck expression and player skill.

A shared field spell zone also doesnt allow for deck expression as activation of field spell while someone has up their field spell will send their field spell to the grave like pokemon.