If you leveled one skill then the cost of Leveling all skills increases instead of just that one skill cost increasing. i.e cost to level up one skill it costs 3 levels, after leveling it up it costs 4 to level up a skill no matter which skill it is. so it costs 10 levels to level a skill up whether it is level 10 or 1.

what im thinking would be nice is to be able to change the base cost of skills via commands so that when a skill is leveled up it changes the base cost of skills.

You can change the base cost of skills using CompatSkills, though I am unsure how things interact when it is done while things are currently running, AND even if it works it would change it for all players if it is on a server as there is no per player level cost concept in Reskillable.

using crafttweaker and event.player.skillData.getSkillInfo(skill:reskillable:agility).getLevel();, you can make the equation of the new base a function based on the specific players levels of his skills. However not sure how i can use crafttweaker to make this work.

You can change a skills base level using the bracket handler for skills, https://github.com/pupnewfster/CraftTweaker-Documentation/blob/compatskills/docs/Mods/CompatSkills/Bracket_Handlers/CTSkill.md but what I meant is it doesn't accept a function it accepts an int. Not to mention things don't really sync from server to client so you need to make sure that it is getting updated on both or weird things may happen.

yep, definitely not going to be able to figure this out. rip