Andy,
Two ideas:
--- First Idea ---
I do like the suggestion you suggested.
Once a country hits 100 million population the "Change workers professions" should be increased by 350,000 per month (this would give countries a 650,000 total exchange rate monthly)
Once a country hits 200 million population the "Change workers professions" and "Exchange with other countries" should be increased by another 350,000 per month (this would give countries a 1,000,000 total exchange rate monthly)
As I have many counties at 100 million or 480 million population.
Realistically I would need to be able to do 500,000 to 1,000,000 in monthly worker conversations just to keep up with basic changes.
---Second Idea---
Is it possible that instead of a hard number (300,000 worker exchanges per month) would it be possible for to code a percentage per month?
Maybe the game allows players to convert 0.005% or 1% of the population per month?
This would allow all nations of any size to convert pop at reasonable levels based on size?