You can improve the raiding possibilities if you want to do the extra work, and where it doesn't conflict with the aesthetic of the map. Orange arrows represent new entrances/ways that should be useful. Green ones are the "important" existing ones.
Then you can make it harder for the defenders to counter raids by removing entrances/ways close to their base (yellow). Entrances further away can be used for defending/attacking both, but if an entrance ends in a base, it can only be used by the defender (unless you want to attack the base itself, but this is very rare), making his ways to intercept raiding units shorter. Little picture to illustrate (green is attacker, attempting to raid area behind red's base):
With all the raiding ways, and especially if you add the new ones, you can place some mex in the backside in the green area in the last picture, maybe something between 4 and 8 per team, in groups of 1 or maximum 2. The optimal number of how many is hard to say and you'll eventually find it by playtesting. You also have the possibility to move 1 or 2 of the "base mex" further away from the spawns, if you want more mex in the backside, but can't add so many before the total mass numbers for the map become too high. I wouldn't do that yet, but it's an easy option to finetune the map later.
Then I pointed out some slight possible balance problems, the top team has shorter and easier ways to reach some equal/contested area. I don't know if its bad enough that it needs to be changed though, and at least for the two expansion mex for the middle guys, the fact that they are placed together might even the longer time to get them out. If not it is easy to even out by placing some of the new (or the 2 "outer base mex") mex for the top middle guy a bit further away than their bottom counterparts.
Then it might be a good idea to move the 3 mex on the left from the mountain into the vale, but testing the map will show if it really is. Just some things to look at when playing/testing it.
And a unrelated side note about mapping, I've seen several times mappers complaining about not being able to use custom decals, but what about the maps who use them, like Aperture or the City Wars maps?