The First Strike command breifing alludes to other, less manuverable bombers than the Athena, and the FS1 intro cutscene seems to indicate a bomber very similar to the GTF Apollo: a two seater with large missile ports. So there were bombers, we just don't know anything about them, I suppose the Osiris and Amun are T-V era.
Another small problem is the lack of weapons, only the ML-16, MX-50, Vasudan Light Laser, Training laser, Harbinger and Fury are canon pre-FS era weapons. We know the technology to make xasers was available, but was expensive, so you could have a heavy cannon based off of the Subach (damage is kind of extreme without shields)