Help with making an rpg stat system

Hello, currently I'm trying to make a pretty standard RPG mod, but unfortunately am stumped by the most basic aspects of the process. I'm looking for a way to increase player stats WITHOUT the use of potion effects, the one that's stumped me the most is HP. I've avoided using health boost because it only gives a minimum of 2 hearts per level, and I haven't found a way to make it permanent, even without those issues though I'd much rather change the player's actual max HP permanently.

I have a leveling system in place, and was hoping to be able to increase the player's HP by 1 full heart for every level, unfortunately I couldn't find any code block for changing the player's max HP, is there any solutions or workarounds to this? My current idea is pretty much just to check if "HP variable" is > current max HP, and if so, change the current max HP, but that last part doesn't seem possible.