[sliken]

One effect not considered in early considerations was the differential radial speed. So even the ability to seed planets 10 light years closer or further from the galactic core over a few 100M years results in expansion much more quickly than earlier models.

[Retric]

If you’re talking 10 light year colonization jumps that takes very deliberate and resource intensive expansion efforts. If you instead model things assuming generation ships as harvesting kepler Belt objects, moving between stars only incidentally or as local resources are exhausted. And only occasionally reproducing. The relative motion between stars becomes the primary driver of net expansion, and that’s quite slow.