Oh, and to add a bit more to that, regarding the gate raiding timing, etc. Here's a theoretical, but fairly typical scenario for me:
Starting planet: 4 hostile wormholes, two easy to take, two high-level.
1. Take the two that are easy to take. Home planet hostile wormholes now down to 2.
2. Gate-raid the two high-level home-adjacent planets next, and kill special forces and (ideally) train stations at that same time.
3. Leave permanent moderate defenses on those wormholes, let's say 40ish turrets and maybe 15ish tractors on each. Consider the home planet completely safe from direct assault, but will add more turrets if needed (and more force fields) to protect against later indirect assault.
Now we're down to our two front-line planets, lets say those each also had 4 wormholes to start, and that we were unlucky and that they did not connect to one another. That means that we now have 6 hostile wormholes facing us in total. Between our mobile fleet being able to move between these planets (and come home from offense when you are threatened), and some light-to-moderate turret defenses on all three hostile wormholes at those two planets, you should be pretty well insulated and not losing any harvesters at all.
Then you've got yourself a little nugget that is pretty well protected, basically. The next decisions depend on the map, and how it grows outward. Such as:
- Is there a planet that is adjacent to both of those two secondary planets you captured? Take it! Unless it is really high level, in which case gate-raid it.
- Is there a planet that is adjacent to one of those planets that is a dead end? Take that! Again, unless it is too overpowered, in which case maybe gate raid it, or leave that as an ingress point, depending.
- Is there a planet that is really highly connected, and borders everything? Either gate raid it, neuter it, or completely take it from the AI but don't build on it if you need to travel through it. The high number of inbound connections can make it not worthwhile to capture, but very worthwhile to take from the AI.
And so on. This is the general decision pattern I typically go through, and believe it or not you can generally scale this outwards without having exponential growth in your surface area -- and without gate-raiding every planet on the map. Gate raids are early and ongoing for me, but I would guess that I don't do more than... I don't know... maybe 15-20 total in an 80 planet map throughout the entire game?
Thus the breakdown of planets at the end of a successful game for me might be:
80 planets total
20-30 planets belonging to my team
0-3 planets that were taken from the AI, but that we did not capture for whatever reason (nice to have buffers sometimes)
15-20 planets that were gate raided, and had their special forces guard posts and train stations killed
27-45 planets that I never touched, except to perhaps pass through with transports or a convoy, or possibly to knowledge raid or kill data centers on.
Out of my 20-30 planets, usually it is good if somewhere between 8-14 of those are reasonably secure from attack by the end of the game. Those are producing resources and not really at a whole lot of risk except when the AI slips past me on the front lines. If I lose some, I rebuild, and if a planet it just too hotly contested I might let it go fallow and just stay neutral.
The remainder of those planets are ones that I captured further out into the galaxy usually for advanced research stations or factories, and I mostly ignore the resources on there, and concentrate defenses in such a way to protect the valuables without having to try to defend all the wormholes individually (which is impossible on those planets, usually). And some of the remainder of those planets were ones that I captured specifically to create staging areas for attacking AI targets. Those sorts of planets also tend to be really isolated, and I protect the stuff on it all in a cluster of turrets and force fields near the command station, but I leave the harvesters to their fate. I guess those planets would be an obvious case for harvester exo-forcefields, but I don't tend to think of that for some reason.