First fix nation population. How can Russia has more population than China. Nation population suppose to be realistic.
Hospital - increase disease resistance
School - for non adult
Barracks - recruit army
Research facilities - for researcher
Change library to research facilities
Population that are not just a number
Add adult (18-60), non-adult (0-17) and retired (60^) pop.
Add Life expectancy - old pop that reach certain age die.
Life expectancy increase base on technology. [ New technology - healthcare ] from 60-100.
Disease decrease life expectancy by few years depend on disease.
Each turn baby born base on growth rate. When they reach age 7 they automatically put in school (if a province have one) until they reach age 17. When they become adult they go to industry or research facility.
Adult - can be recruited as army, can work (workshop (economy) and research facility ( research)
Retired - cannot be recruited as army. Not all can work, if they can it only give small economy output.
Non-adult (Children and teenager) - cannot work and be recruited as army. They need to go to school. Each province need to build school so when they become adult it will increase economy and research. Province that that doesn't have school produce no research when they become adult and if you build
Only adult can be recruited.
Province that doesn't have barracks can recruit army but it has -20% attack and defense because you not recruiting a soldier but a normal civilian. It also have -20% cost and upkeep.
Army cannot be recruited if there are no adult in a province.
Reduce growth rate, Immigration and increase emigration.
When army battle in a province it increase devastation by 2%.
Plunder increase devastation by 20%
Horde can massacre population - kill 50% of a province pop and increase devastation by 50%. 10 years cooldown.
100% devastation have no growth rate and have 100% emigration, damaged buildings (can be repaired).
Each capital have immigration attraction except tribe and horde.
Immigration add pop base on emigration% of each province in a nation to capital every year.
New diplomacy - Immigration treaty
Immigration come from outside of your nation base on opinion of that nation.
Devastation increase emigration.
Disease increases emigration.
Immigration increase growth rate.
If a province has no adult it decrease growth rate by 100%. Growth rate will increase slowly when the children become adult or when people immigrate.
Devastation decrease growth rate.
Only adult can work.
Workshop increase economy.
Research facility increase research.
Happiness affect work and immigration.
-1% happiness = -1% economy and research = +1% emigration = +1% revolts risk (if you have other nation core on a province)
If this is too much or too complicated at least change happiness. Happiness barely do anything. The reasons why I suggest this is because recruiting army reduce population significantly and for this reason I don't recruit army in capital so I can focus on economy and research in capital.
Likely trade accept % is always at 0, so you never know if the AI is likely to accept your trade or not. Internally it isn't at 0%, but the GUI will always show it at 0, no matter what. Also you can literally make countries declare war on an another country without giving them anything, and they'll accept the 100% of the times. Russia just declared war on you? No worries, just make every single bordering nation declare war on them.