Here are some mods I can recommend that directly affect game balance, feel free to suggest more. For the most part I've listed only things that directly affect the basic variables in simple ways, not including survival mode stuff, combat overhauls, perk tree changes, the paraglider, and many other things that obviously do also affect game balance but aren't only about that.
*Skyrim Skill Uncapper for SE and AE - Adjust rate of level up for each skill individually. I like level-ups to be way slower than the base game, like 10 times slower, but with skills where tedious grinding is already the only way to train them left closer to normal but contributing less to character level advancement. Enjoy being level 1 for a meaningful amount of time.
*Armor Rating Overhaul - Makes early-game armour not completely useless. Getting some should be the priority it normally is in such games. The base game armor rating algorithm seems weirdly broken, any of this mod's options is better.
*Yet Another Difficulty Mod - Adjust damage dealt/damage taken, optionally changing it automatically as you advance. In my current game it's set to start with 1.0/2.0 at level 1 and smoothly progress to 0.5/5.0 at level 30 (compared to 0.25/3.0 for default legendary mode) and so far that seems good.
*Extra Encounters Reborn - Encounter more problems when wandering around the world.
*High Level Enemies - Make those you encounter more dangerous. Many of the people and animals who live only to murder passing strangers scale to higher levels than previously possible.
*NPC Regen Nerfed - Taking away their infinite magicka supply makes things a little easier.
*Simple No Health Regen - If you don't go full survival mode, maybe at least have no health regen.
*Smart NPC Potions - They might use two or three healing potions during a fight. Also they might poison you.
*Maxsu Combat Escape - When fights are actually difficult you might want to be able to run away without that stupid ogre continuing to be angry at you forever from the far side of a mountain.
*Simply Better Movement Speeds - My current choice of movement speed mod.
*Encounter Zones Unlocked - Probably helps if you're going to have a playthrough that goes on for a while.
The most annoying problem with RPGs when you finally get to a high level and stuff is there's never enough enemies to challenge you. Thanks