After a couple weeks of playing (not counting the week+ I lost electricity), Space stage still feels like the focus of Spore. And after three tries, I've finally found some strategies that prevent hectic overload and allow me to progress at a decent pace.
These tips are advantageous, but whether or not they equal fun depends on your own particular tastes. So think of these as strategies to try out, rather than the way the game is supposed to be played.
LIMIT YOUR CONTACTS
You don't have to make contact with every alien species you find. Whenever you make contact with a species, you can see every star system in their empire by toggling Empires in the bottom-left corner in the galactic view. Any inhabited stars outside the empires you see in this view belong to species you have not contacted yet.
By introducing yourself to new species deliberately and paying attention to whether or not each star system is already inhabited, you can control the pace of your game to suit your own personal style. It might take a couple play-throughs (different species of your own) for you to find the pace that feels natural to you.
Don't think you have to accept every mission that's offered to you. If a friendly species asks you to destroy the turrets of their enemy, an enemy on a planet you haven't explored yet or one of your allies, taking that mission might involve you in a war. Fight one war at a time, or avoid them altogether.
Be aware that your allies might go to war with each other. If they do and they call for you to save them, you can choose to ignore the request if you want to keep both allies. Ignoring distress signals will eventually lower your standing with an ally, but it won't break the alliance immediately (at least, not on the lower difficulty levels). You can make up for it by performing missions for that ally.
WAR COMES FIRST
...which doesn't mean you should start the Space stage by blowing ships and colonies to smithereens (though you certainly can if you want to). No, I mean that once you start a war, stick to it until it's done. Otherwise, you'll keep getting called back from other things to protect one of your colonies or allies from invaders. Also, the enemy will expand its empire, thereby forcing the war to continue that much longer. Strike first, strike hard, and don't relent until the enemy's empire is eliminated.
As the great general George Patton once said: "Fixed fortifications are a monument to the stupidity of man." The best generals always stay on offense.
MAKE ONE SYSTEM YOUR MONEY-MAKER
Once you've got your terraforming skills up, search for a solar system with four or five planets and two or three types of spice (blue, yellow, etc). Terraform them all to T3 planets, place the maximum number of colonies on each, and raise each colony to maximum production by installing factories and homes (I add one entertainment building as well... but I've never had a colony revolt on me, so I'm not sure if this is necessary). When you're done, you can simply hop from planet to planet within that one solar system and sell their spices to each other to make money quickly and easily.
Yes, terraforming, colonizing, and building all this is expensive. But it's well worth the investment. My system has five planets with purple, green, and blue spice... and I can make literally millions in under a minute. With three colonies on each planet and all those colonies completely built up, by the time I've collected the spice from all five planets, the first planet already has more for me to pick up. Just pay attention to each planet's prices, so you know where to sell. The highest price I've seen so far was $60k per spice crate.
DON'T TRUST YOUR OWN
Sadly, your own people will charge you high prices for some or all items. My religious herbivore colonies charge me more for colonization packs than any of my allies.
Prices seem to reflect species specialization. A battle-oriented ally of mine charges me literally half the price for weapon and defense technology than my own people charge me. Take a few minutes to compare whole species in prices. Then, if you care to, you compare at the level of individual colonies. A colony's prices for spice do not always remain constant, so pay attention to price every time you sell.
If you missed my earlier Spore tips, you can find them here. As usual, I welcome any input. If you have any tips of your own, please share them in a comment.