Phase Through Blocks

So long story short I'm trying to make and ability that allows a player to sink into the ground/go through walls while being able to move freely inside. it would function sorta like no-cliping or spectator mode, but i wanna refrain from using that cuz it would mess up other things in my mod. while my original plan was to use spectator mode i realized the mobs in my mod would stop targeting the player for attack, if there's a way to fix that then I'm free to use spectator mode. is there a to do this with as little code as possible? also this im sure requires a lot of code but is there a way to make the player sorta swim through blocks?

Example sorta