ORIGINAL: Malevolence
How does this impact the mid- and late- game play? Is the planet essentially conquered before later faction developments?
The short answer is yes and no. You can have games where that happens, you can have games where that doesn't happen.
It depends on the size of the planet/map, the terrain, the wildlife, the research speed, the economy, population, deposit location, the infrastructure that survived the apocalypse, how lucky you are in finding republic equipment, luck of the dice, luck of the neighbors and the difficulty. The really big selling point is that you can have a lot of variations in your starts, even if you have to reroll a lot to get something that you like.
Early turns happen fast, once you get the original organization out of the way, the reason why let's players take forever to do the turns is because they have to explain game mechanics to people with no access to the game. They also tend to play on small or at least normal maps to keep the campaign short.
The bigger the map, the higher number of major regimes that equal the player in capabilities. The bigger the map the longer it takes for the player to expand everywhere and the more easy conquest is denied by the expanding major regimes. The more mountains and seas on the map the harder it is to get around as chokepoints have to be overwhelmed in frontal attacks while crossing mountains and supplying over them is very hard. Meanwhile on open plains motorized units can take huge swaths of land with little difficulty, at the same time the AI can do the same to you on the plains. If you have aggressive wildlife that'll slow you down or even ruin your start if you can't contain it. If you get unlucky and the minors around you happen to be the very aggressive slavers or raiders you'll struggle whereas farmer regimes will be pushovers.
IMO there's 5 stages in the game:
1) Starting expansion where you basically poke around with your troops to see what's to seize for your self and how much of it you can before you need to invest into armies. Depending on your luck this can be a lot without problems or barely anything as the wildlife and the freefolk give no quarters.
2) Taking on the minors, usually as a result of encountering some limits, be it distance or being boxed in by said minors, this is where you usually expand your armies if the freefolk didn't force you to do it. It's also when you'll get your second city.
3) Dealing with majors, where you'll be fighting people who have real professional soldiers, and similar if not superior abilities. This is where you get into the real wars and the need for proper strategic planing for conventional war.
4) MAD warfare, where it starts to get freaky and deadly with mobiles shields, plasma tanks, AI armies, mobile automated pilboxes, mega-tanks and nuclear strikes.
5) End game cleaning. As with all games once you get powerful enough nothing can oppose you so it's just uninteresting busy work, clearing up the last opposition from whatever cave they've hidden in, final touches on the map painting, crossing your T's, dotting the I's.
All of those stages can be very different. I've had very easy starts where I've expanded fast because of weak neighbors and defensible terrain, I've had abandoned starts where I was pushed back and boxed in immediately with little chance of success. I've had good starts ruined by horrible dice rolls on events (got my economy crippled because slavers stole half my workers), games saved by good rolls (getting a tactical nuclear warhead at the right time to use against an enemy doom stack).
If you take the time to tweak the game settings you can make it offer a lot of what you want.