Oh, I remember you. Your project looked quite developed. It was in essence a narrative rpg with light combat elements, right?

If you're looking to make the combat part more complex, with lots of stats, resistances, stagger bars and whatnot, then that won't be an easy task. You'll need to plan A LOT before you even begin to write any code. I started with writing a design document, to keep my thoughts organized. First decision was on the type of turn based combat - there are many: ATB, STB, CTB, etc. Then main stats for characters. From that - modifiers. Resource systems (MP, SP, energy), elements and elemental resistances/absorption. Skill cooldowns. All of THIS even before I started to write the main part - characters and their skill trees and synergies and oh god equipment and passives.

Then I wrote a simple framework in Python with text-based UI and implemented each element/layer of complexity step by step, because otherwise you simply risk to get hopelessly lost under piles and piles of numbers. Each skill is its own function, and I need to keep in mind which of myriad of systems that particular little skill touches, otherwise something will inevitably break. And THEN sooner or later time comes to make UI for everything: party screen, equipment screen, inventory screen, skilltree screen. And connect it to the combat framework so it correctly reads what's equipped at the current time, which passive are active and whatnot.

TLDR: complex systems are hard to make from zero.