As the others members above have pointed out:
1. Your M904e is an excellent bit of kit that does all you ask of it.
2. Deep down, all airsoft guns are the same, they fling bb's, some marginally better than others but throwing cash at them doesn't always guarantee improvement.
I agree you should have a backup gun, & yes it makes sense that it should have magazine compatibility with your 904, especially if your loadout limits the size of mags you can carry.
the obvious plus to this is if one plays up mid game, it's a two minute job to leg it to the safezone to grab the spare, no fuss or time lost ๐.
Your 904 is a stubby gun, why not consider one of the longer variants, 907h for example, set it up as a dmr, pretty it up with magnified sights etc, gives you another style of play on a platform that your familiar with, it's what I'd do, especially if my LCE restricted what mags I could carry.
alternatively, if that's not the case, get what you like the look of (subject to positive reviews), me, I'm a sucker for the longer battle rifles, m14/G3/SLR, whatever floats your boat ๐ค
PS @Rogerborg I'm nicking that picture, awesome, the truth will set them free lol ๐