Looking good! A few comments, from my own experience at least (I'm the developer, but opinions vary):
Generally speaking, unless things are going poorly, I tend to take around 1 planet every 45-60 minutes or so. From that standpoint, you're really doing a ton of consolidation and the main reason that you're sitting at so high of resources is that you aren't actually building enough ships. Even if you're building tons of turrets and such to defend your own lands, you should still be cranking out tons of mobile ships and sending those against the AI planets to capture yet more territory. Of course this then requires more defenses and more gate raids, and so that then requires yet more resource output and thus more resource input.
Having larger active fleets also requires more energy, which then requires more resources, which requires taking more planets, which requires more fleet ships... it's a vicious cycle. Only reason you should ever have 600k of your resources is if you're already at your ship caps. And if you're at your ship caps and not being viciously attacked, you ought to be moving out into the galaxy to make use of the fact that "you have the tempo" (to use the Chess term) and can press your advantage. Depending on how your assaults go, you may lose the tempo and have to regroup or rebuild, or you might go snowballing forward. If you're always able to snowball forward and still have tons of resources, then the difficulty is definitely too easy.
Generally speaking, eventually you'll hit some roadblock planets that you can't get past easily, and then you have to think outside the box and do multiple strategic hits against it, or unlock some special ship, or whatever. Knowledge is a key growth limiter, too, since unlocking more ships increases your ship caps, too. That then lets you spend more metal/crystal, incidentally, and makes it more expensive to rebuild your "entire fleet" if you lose it all, since that then comprises more ships as you go.
Assuming that everything is pitch-perfect and you are comfortable and the AI is right at the level where it is just your difficulty, that's what you will generally see, anyway. If it is too easy, you'll snowball the AI and always have lots of excess resources. If it is too difficult, you'll always be resource poor and will have trouble taking many planets. The game tends to self-balance to a certain extent, though, since if you go snowballing you might overextend yourself and wind up with an AI progress that is high enough that the AI can do some surprising things against you (sometimes resulting in your loss when you think the current scenario is a piece of cake, which is interesting).
In the case of my current 4player game, the AI is really giving us the runaround, on the other end of things -- we're playing against two 7s, our usual, but the current map has been really tough. We've lost 3 of our 4 home planets, and routinely lose 6-7 of our 10 planets in the AI waves. But, we're gradually clawing out way outward and taking new territory and getting on a bit of more solid ground. As long as we don't lose that last home planet, we have a pretty decent shot of making a surprise come-from-behind victory, I'd say. But you just never know. Those surprise wins can happen for the players or the AI, so it's always good to be vigilant in any game.
If you're sitting that pretty in your current situation, though, I'd say it is time to get out and kick some butt and get some more planets. Before long, you'll have more territory but will also be in a more difficult spot to maintain. Depending on how defensive you are, when you start feeling more put-upon you can stop expanding and refortify. I'm a defensive player, too, but just bear in mind that the main thing you need to protect is your home planet: so the more controlled-by-you planets you can put around it, the safer that home planet is. Sometimes the best defense is indeed a good offense in terms of taking more buffer planets to put yourself at ease.
Of course, that's just my own playstyle, and part of why I play difficulty 7 instead of something higher. Other players that are by nature more aggressive and are comfortable living on the edge more in terms of having less buffer and fewer planets, tend to play on diff 8 or more and use wildly different tactics. But as someone more defensive in nature, you might prefer doing it more like I tend to (or some variant that is unique to you, anyway). I tend to take around 20-30 planets out of an 80-planet map, and I tend to take around 1 planet every 45-60 minutes, as previously noted. That tends to lead to a balance of offense and defense that I'm usually pretty comfortable with, and I win about 80%-90% of the time at diff 7, although there are very close calls quite often, and in multiplayer we almost never keep all of our home planets unscathed (if we keep half of them, we're doing good).
Anyway, you'll find your own preferred way of playing simply by doing what feels natural to you, but if you have a ton of excess resources you know what to do: spend them! If you can't spend them because you're at ship cap, then attack so that your ship cap goes down. With that many resources, you can afford some engineers and docks to replenish those destroyed ships very quickly, so it won't even create a gap in your defenses, which lets you keep feeling safe: as you lose ships abroad, replacements just get rebuilt at your own planets and can either defend or go and join the front.
Glad you're enjoying the game!
