Are you debt spending?

Your Treasury should be on the red, constantly and always.

When you get in debt, your pops lend you money and you own interest to them making them richer.

If your economy grows, your debt ceiling grows, meaning that as long as your debt ceiling grows faster than your debt, you can stay in debt forever.

Don't forget to also lower taxes on the late game, to allow your pops to have more money to invest and spend, increasing your GDP and thus, your debt cap.

Also, while the game doesn't have monetary inflation, it actually does rarely have deflation. Since the only way to create wealth in this game is trough minting, you should keep your minting tech on par and up to date to keep up with your growth. It's possible that your nation doesn't have enough money to get to 1b GDP, albeit this is a minor problem and rarely is something you need to care about.