Simply go to
"\steamapps\common\Stellaris\common\defines\00_defines.txt" in your files.

use ctrl + f to find
"LEADER_CAPACITY_XP_SCALE"

To the right there should be a number (.85) change that number to 1 and you will never be penalized for going over your leader cap.

Or switch it to a larger number and you'll be rewarded for going over leader capacity, I don't judge ¯\_(ツ)_/¯
have fun exploring the new astral rifts o7

This will only change the EXP penalty for going over leader capacity, you will still have increased unity costs per leader over cap.

Also any AI empires will also get the benefits of whatever you do

If anyone knows where to change the unity cost of going over leader cap please drop it in the comments or hmu on discord @damp_tostr