Fixed it. Responses helped a lot for the process, big thanks to those who helped out.
Don't know if you can tell over the photo but the light is as good as ags-101's low option now (maybe even a bit brighter). There's a little story though. I finally received the needed screwdrivers, tried the potentiometer adjustment initially. That changed nothing sadly, the adjustment's best result was the brightness level I already had on the screen, therefore, I thought the screen itself might have been the problem.
I disassembled my ags-101 and took out the screen, did the same to the agb-001, and swapped them, using the screen I knew that it was the genuine product hundred percent. Voila! The light is great now. (The lesson to get from it is that you shouldn't prefer buying a pre-modded gba from Zerey. I guess there have been satisfied buyers but I don't know, my purchase had troubles... Just buying the mod cable is a better way to go)
Anyway there are things those puzzle me actually here. I used the Zerey's screen on my SP since it was screenless then, and the brightness level was perfectly fine, with both levels. I was expecting that actually, since that device's voltage structure is made for that kind of screen. But I'm curious about the Zerey's screen's real problem, you think is it a knock-off screen (even if it is, the display quality is perfect, just like the original, sharp looking, correct resolution) or were there different models of screens those were being used on SP manufacturings?
I see that there's a 32pin/40pin talk about the connection cables also. So I'm pretty confused about that, if anyone's there to enlighten me, will be much appreciated. Is the cable type (pin difference) optional or you pick it by the certain manufacturing of GBA you have or its screen?