So, I posted an earlier strat on the Spore rush, here:
https://forums.galciv2.com/?forumid=357&aid=143163#1120072

But I discovered another Spore strat by accident--this is a late-game use of Spore. What happened was, I'm on Maso, I already had 81% of the galaxy under my influence, and I was on my way to winning. I was 9 turns into my influence victory, when guess what: *THE TURN* before I was going to win, the Torians decide to declare war! Now I can't win an influence victory because I'm at war. Bastards! Stupid little goody-too-shoes E.T.'s. Go fly in your UFO's and eat your Reese's Pieces. I had almost no military (takes too much micro to build when you're going to win anyway).

However, it was late in the game, and I was running quite a surplus in my economy. I had 20000bc in pocket. As it turns out, a speed-1 spore ship only costs 1000bc to rush-buy, and a speed-3 spore only costs 1300. Even less than that if you decide to rush-buy on credit (it's late game anyway). You can rush-buy spore ships on your newly-conquered planets.

That's the trick. You want to talk about blitzkrieg--all you have to do is spore one of their planets. And you can do that by surprise attack, if you're the one who's starting the war. Then you can domino the rest of them, by rush-buying more spore ships from that planet. You sneak in a fleet or powerful ship and take out the orbiters, spore one of his planets on his home turf; and, if need be, rush-buy a starport for a whopping 156bc on that new planet. Then rush-buy spore ships from there. Pick another nearby planet of his, send your fleet and rush-bought spore ship in, repeat. They fall like dominos. No worries about population--your late-game economy can absorb it--no worries about soldiering, no worries about ferrying population in from your other planets, no worries about building up a huge invasion force, no worries about protecting your invasion lanes where your transports are travelling. You just take out planet, rush-buy a spore ship from it, hop on to the next planet. Now you've got TWO planets that can build ships. Repeat.

You just cut off his head. Now you can worry about fixing your economy, taking on his fleets, etc.--and notice I'm basically taking on his fleets AFTER he's dead. What I will do in the end is take maybe 50% of his planets (the best ones) and let the rest flip to me. And note that, on high difficulty levels, you're stealing his techs on that 50%.

Man, spore is fun.