I would suggest a decent bit more attack damage for leaf cutter media, to incentivize building more media for the purpose of damage dealing, and then building majors to do the tanking. As of right now, majors fill the role of both primary damage dealer and tank. That's fine, since it is realistic.
However, I think that media could still do with a damage buff, since dealing damage is probably the leaf cutter's biggest area of weakness. The level of difficulty of 3.1 and 3.2 is a significant bit higher than 2.1 and 2.2, and maybe giving media more damage would help make the level feel fairer to newer players, as insect spawns would be easier to deal with. The same goes for free play.
If you don't want to possibly throw off the balance of 3.1 and 3.2 too much, one option is to only give leaf cutter media a decent attack damage buff at level 3 only, since it is unlikely people will upgrade their ants in 3.1 and 3.2. And I think this would work well with the challenge modes on 3.1 and 3.2, since I feel like there should be more benefits to upgrading leaf cutters to level 3 on these levels.