Instead of giving the AI a resource bonus, why not give the AIs a hull/shield bonus?
For example, easy: -10%, normal 0%, hard +10%, unfair +20%, vicious +30%, cruel +40%.
Of course, this should be in combination with a slightly smarter economic planning (namely that they invest in starbases with cargo bays).
And why do I suggest this ... currently, an AI has a really big economic advantage, which it can use to expand its empire and on top of that to build a large fleet. Basically it's an exponential growth model (for a while at least) and the AI has a big starting advantage. Because it's exponential, the difference with a human player becomes very, very large after a while. However, much later in the game when the AI and the human player have hit the research and fleet limits, there's nothng the AI (or human) can do with their resources anymore ... and then the AI has lost just about all of its advantage. It still has a good income of course, which it can use to rebuild its fleet quickly, but so does the human player.
The only thing that needs to be added to the AI is a small subroutine that manages heavy constructors and starbases. Whenever there's a new colony, send a heavy constructor there, build a starbase there and upgrade with level 1 cargo bays (at the very least). They should also send such starbases to gas giants, asteroid belts, and the star. To keep it simple, you could limit this to locations that are within range of the culture.
I expect that this will make the difficulty of the AIs easier to manage and easier to understand for players... because the differences in difficulty would be more consistent throughout the game, from start to end.
In theory at least.