How did you build the planes?
The aerodrome district is the first thing barbs/GDRs will pillage...on Deity anyway.
I'm playing the official Emperor, not the modded diety version, the GDR's have some sort of custom script and in both games didn't pillage my aerodrome (even if they did, it's one turn to build a new one or repair it), not sure if that was luck, or scripting, they also didn't really attack my cities, so I didn't build encampments in my cities except in pinch points or first landing on a new continent?
Once I found a neighbour (Sent a rockband East and a Naturalist West) I sold my oil (and everything else) and saved to buy a bi-plane (1440 I if I recall correctly). I should have deleted the starting military unit, he was useless and costs gpt.
I took the Settlement pantheon turn one and built a city to the south before the GDR came north, I placed an encampment in the north in the capital which seemed to keep the north safe. Then I built a government plaza in the capital with ancestral hall and magnus to start pumping settlers.
City two built an emcamptment and theater square, then built only settlers to start, once you get a city north of Ottawa the GDR threat is mostly neutralized, it will stick around that city forever getting stung to death by the new cities encampment. There are two GDR's in the south, but I was able to contain one in the far west section of the continent and filled out the rest of my peninsula. The north took much longer as the GDR's would run away eventually, or pillage to recover health. The planes were the best way to kill them quickly.
I had nearly every city build a theater square and harbour as it's first two builds, I wanted to rush Aerospace Contractors for the +3 Aluminium per spaceport (doesn't require pop). Realistically you only need a handful of aluminum.
I don't think pop really matters as districts are free and come with a building, they also repair for (free?) cheap, so I built a hockey rink in every town, then if I could maintain 7 pop, I let the builder do whatever was most interesting, rarely that was a farm.
it's cheaper and faster to build a settler than it is to gain +6 pop to hold 2 additional districts, my land was full anyways thanks to the burnt woods and archeology sites (which some of which don't work, forcing you to dig up something else with the same archeologist later)
Then it's just a matter of zerg rushing settlers, the free GDR is good to pick up the settlers the AI lost to the barbs, so send it around the world, I planted 3 cities on the last turn all from Korea's land.