This Seeking Tech guide will explain how you can get free Luck bonuses without having to level up or spending half a day in Metaphor ReFantazio.
After you unlock the Gauntlet Runner, you need to wait until it is Idelsday, which is the day that ends with a 0 or 5.
Afterwards, make your way to the toilet to get your free Luck bonus. Do this every time during Idelsday to boost your Luck stat.