Germanics were never a horde....

The Germanic tribes/people, like many pre-Christian especially pre Roman Northern Europeans, generally had more monogamous among lower and middle classes. Concubines(if foreign women usually captured in war) and polygamy(native or high class women) being more of upper class benefit. They are not popping out kids like Turks, Huns, or Mongols hordes did.

Most Germanic men and women married around their 20s to have kids. The main reason they probably had Germanic tribes be more quality over numbers is because one they do have people at home farming or living off land even if semi nomadic which means less out raiding but better fit to fight off invasions. Two monogamous relationships usually means each kid gets more investment on behalf of father instead of one who had bunch of kids with different women. The Germanic tribes were very militaristic and conservative/uptight society but not horde of horsemen

"This looks great! Would you consider removing the "migration only during peace" condition for gauls and germans?"

This looks great! Would you consider removing the "migration only during peace" condition for gauls and germans?

yeah just anything to make it feel gradual. Totally up to you of course :)

I could make an event that gives you a buff if you're at war for a couple years?
That buff could go away after a timer or when the war is over

sounds great, but is it possible to trigger a timed event to lose the buff instead of instant?

That’s a good point
Maybe I should include a manpower modifier or tribesman buff then...
Or I could add some events to get extra manpower

Sounds nice and all, But when you have this you as a result have low man power. Meaning that all it takes is one major battle to completely outcast a migrating tribe. Or cause its destruction.