With time, effort and money you can tweak / get a standard 1.14 Joule AEG to shoot quite far if using a decent BB weight like 0.32g, certainly to the point where you can stand 10 meters further away than other players on your teams and shoot the exact opposition that they're engaging and know that most of the opposition players BB's will fall short and not get you. If you go on YouTube "Negative Airsoft" is spot on when he says in one video from 2023 that you can build an all purpose battle rifle that can be used for CQB and then the next moment be used as a half way house to a DMR when outside (it's what my M904E is now).
I do enjoy having my DMR though, especially as I'm 50 / not that fit, and I'll play the morning with it then swap to my M904E after lunch. The major downside of running a DMR though is that you MUST have a sidearm as you'll only be able to shoot at people if they're over a certain distance away from you (MED, aka "Minimum Engagement Distance"). So, once say an enemy gets closer than 15 meters to you you'll need to use your sidearm / pistol to shoot them as getting hit close range with a heavy BB bloody hurts and will draw blood even through clothing. My sidearm is a mildly upgraded AAP-01.
Others have recommended parts from AK2M4, and this parts supplier is damned good and is happy to answer questions, so if as your post all you want is to shoot at around 400 FPS on 0.20g BB's with your M908A then I've a feeling that a GearParts / XT M120 spring will be the safest option for you (possibly an M130, but if one day you're building a DMR I'd buy both anyhow as they're so cheap). This will give you the raw power, but your air volume etc. will not be high enough, so don't expect to become a lethal killer on the field
Also the inner barrel you have is good enough, you don't really need to replace it, but I did with mine as I took it to the next level and wanted a Maple Leaf inner barrel to get a 100% perfect fit with the ML MR bucking (which with the right nub is a simple to install R-Hop).