Is there any way to shop after the start of the Depths of Mementos dungeon?
Solution 1:
As far as I could see, there is no way to go out and buy items after you started the Depths of Mementos.
But, when you are starting the Final Palace, there is the girl in blue next to the velvet room door that can give you a full HP/SP Heal anytime you want.
You can also teleport from the door to the final boss back to the start of the Final Palace, where you can do the full heal before starting the final boss.