Well I noticed a lot of people saying how defense is useless against a vast fleet, which is true.
And the way it's designed to be

Defenses were never meant to be strong enough to beat back large invasions. In the case of AI, you can actually use defenses to beat up a disproportionately large fleet. Humans will of course be able to counter this (the AI doesn't target construction ships, which means you can rebuild 4 turrets in the middle of battle), but nonetheless

One thing that gets me is how drastically upgrading your fleet supply decreases your income. I think it should scale to the amount of ships you have, like if you max your fleet supply at X level, the correct deduction from your income should be made. So people with max fleet supply won't have -75% income at all times, only when they max it out.
When you research fleet supply, you're basically providing the infrastructure to support the fleet. It's not the direct fleet upkeep costs, but how much it costs to set up and keep the necessary infrastructure functioning - can't really reverse that

It was done this way to get players to actually decide when they need to upgrade and to live with the consequences, otherwise it would just be too easy.