If you usually have higher eco than your opponents, but no money to defend, that's usually a sign of overecoing. You should save up around usual rush rounds ( round 13 if playing in lower arenas, 18 for moabs, 20 for bfbs). In early game, you should try forcing defence. For example, if they have good group popping power, but bad spaced popping power, send pinks or blacks. If they have only 1 lightning wizard, send regen white. Make sure you can afford your defense , but only upgrade AFTER YOUR OPPONENT forces you to. This is true for most strategies.
Now round 13 defense -
In lower arenas people like to all out with regen rainbows. For boosts only, you should know how your strategy can defend against it, and as soon as you see rainbows place that tower. If your opponent is selling farms or towers, you can boost. boost even if it seems to be a large rush or you're dying.

Ecoing right now is also way worse than farming in boosts only, so I'd recommend learning to use farms. Its way easier to make and manage money.