While I like Dominators and find they are strong ships once they have a bit of maneuverability boost from skills/hullmods, Onslaughts outclass them by a large margin. Range, firing arcs, gun firepower, force concentration, officer concentration, etc all favor the Onslaught. Dominators have a decent logistical advantage, in burn speed and operating costs; that might matter to a player or might not.
The officer concentration is big if you're worried about being outnumbered, because you can put the "saved" officer onto a smaller ship and use it either as an escort or as a hunter/killer, depending on what kind of ships you're using.
You mentioned in your original post that the Dominator catches ships while the Onslaught doesn't. That is really weird, because a Dom is speed 30 vs an Onslaught speed 25. That's not a meaningful difference. Saying that a Dominator can kill anything faster than an Onslaught is also strange, because Onslaughts have far more firepower pointed in a much wider direction, with more flux to support it, on top of more range and more missiles to back them up. The Dominator has to point at its target; Onslaughts don't.
To be blunt, it sounds like you're building bad Onslaughts. The comments about leaving missiles empty and frigates harassing it are not great signs; the missiles are very powerful and an Onslaught has more OP to spare than a Dominator, so why would the latter be able to afford them and not the former? Frigates shouldn't be able to approach an Onslaught without taking enough damage to push them back, unless the Onslaught is already fluxed out from fighting another capital. Are you not mounting anything but PD weapons on the sides/rear? An Onslaught can have at least a large + 2 mediums firing in something like a 300 degree arc (with more firepower in most of it), with 2 mediums firing straight back, while still maintaining 360 PD coverage from other mounts.