Just a guess on attacking AI bombards: cloaking starship + transports + whatever fleet ship (preferably cheap) you have that kills bombards.
Or no joy?
sounds intensely micro heavy for a dozen bombards. Besides, normally the bombards have a few dozen ships at the wormhole entrance, blowing up the cloaker starships at least.
But I don't think it is a problem. That is their niche. They shoot at you and get a free shot or two before another ship can fire back. They are slow and fragile and take a very long while before shots. Don't make them same-y. The fact they are extra ordinary from the average ship is why they are desired today.
Their niche you say..
Bombards in question have 1.2mil hp (Mk II), they have 33k range which quite exceeds range of ships that shoot at them (can get quite more than one shot even with 30 sec reload) and group speed of my fleet is at 18 and any ships that abandon the group are eaten quite quickly 
Considering that you would need to micro to hunt them on any planet you enter or suffer attrition.. their role doesn't seem like niche. It is simply annoying. Their habit of murdering things like transports, bombard ships and raid ships doesn't make it easier. And AI likes to have them quite more than their cap on given planet.
So would you suggest that their damage go down some? Their fire rate go down some (aka. their reload time go up some)? Their range go down some? Their health go down some? Or their speed go down some (to reduce how well they can kite)? Or some combination of the above?
Although keeping things together is very useful, sometimes you need that speed. Maybe you can instead group move only some of your faster ships (like speed 40ish and up?) together after the bombards. That way, there still is a blob that is harder to pick off, but not one quite so agonizingly slow (18 speed won't always cut it). Artillery ships, by their nature, are excellent on defense. And units that are great on defense will sometimes need a bit a micro to take care of them, because you don't have much of an advantage when you first pop in otherwise.
And, as I stated before, you can abuse bombards in a similar way when using them for defense on your planets (except for the more than ship cap part, of course)
Though, come to think of it, the AI seems to get above ship caps of low ship cap units very quickly. Are you sure the scaling of ship caps and reinforcement multipliers is working right? Sure, the AI gets bombards at a much slower rate than they get, say, fighters, but it still doesn't seem slow enough for their low ship cap. (of course, that could just me being annoyed, and the system is actually working just fine)
