Just wanted to say a big thanks for the pi-hat part! Started working on one today!
On the edge of off topic, what is the best way of routing from the outer edge of the pins... try to fit them between the inner pins?
Whatever way makes the routing easiest and satisfies the drc requirements (and the fab house's requirements) is good. Between the pins usually has the least space and takes the most care to route (i.e. you likely need to reduce the grid size to get enough clearance) but as long as the spacing is enough for your fab house that's fine.