Early game bonus is what leads to mid-lategame power. And the later the game, the less "Artifical stupidity offset" is nessesary. The income already aquired will be enough.
At least intuitively, I would asume decaying would be the better approach.
I'd say the exact opposite.
Let's assume the ressource Bonus goes from 0 to 100.
Now a ramping Bonus would maybe give the AI 0 for the first 10 turns, then 10 for the next 10 turns, then 20 and so on.
So it'd be as limited as the Player in the early game. They can't buy faction Units early, spam colonizers or anything.
The longer the game goes, the Player will naturally be more effective than the AI (the reason the AI Needs those extra ressources in the first place).
As the Player will thus snowball harder than the AI once the more effective early and midgame come to fruition, the AI gets a bigger and bigger Bonus to aid its snowballing.
A decaying Bonus would give the AI 100 for the first 10 turns, then 90 etc.
So it could spam colonizers, faction Units, armies etc., leading to the current Problem that it's almost impossible to get any Goals as pioneer, as the AI doesn't have to wait for cosmite or influence ever, and can regularly insta-buy production.
This would technically lead to a harder snowball in the midgame, one the Player can never outspeed because he simply is limited by his ressources, and the only reason the AI doesn't stomp the Player into the ground by sheer number and power of modded Units is its stupidity to not attack most of the time.
So a decaying Bonus would give the Player a massive disadvantage in the early-game, Balance it out in the midgame, and a massive Advantage in the lategame (unless the AI somehow managed to cripple the Player).
While a ramping up Bonus would ideally Offset the AI stupidity more and more, the more complicated the turns become and thus the AI will fall farther and farther behind without help, as Efficiency becomes more and more important.