Yes, I thought I'd be invincible too. Unfortunately, you're not invincible - not even with $500,000. I got to $500,000 four times (should have had at least $4 million, if it wasn't for that damn limit) and died with $468,000 sitting uselessly in my "account".
The problem is that you can only buy four lives and four shields - and after level 400, the alien bullets are super-fast. You have to get under the aliens to kill them, but the bullets are flying down like rain. Then you also have to avoid the crap weapons and death they keep dropping - which, by level 400, are also super-fast, making them nearly impossible to avoid. On top of that, the green UFO comes out on every single level - sometimes two at once! If you lose your four shields - easily done over 3 levels when you can't top them up - then die, you lose autoshoot and are down to puny weapons that can't kill anything. (even $2000 laser is a puny weapon on high levels when you don't have autoshoot)
I seriously doubt you got higher than 200 million in 5 hours. I played for over 6 hours and got rank bonus of 1 million points every 4 levels. I did everything possible to increase points. I don't think it's possible to get higher in less time.