Don't forget fortresses. Especially the higher mark ones, or the superfort.
Granted, fortresses in AI hands have a pretty chunky range nerf (except for AI superforts)
Also, some of the "brutal picks" for AI homeworld are capable of destroying entire armadas very, very quickly.
That said, if we want to keep the OMDs range, maybe a theoretical Mk 2.25 antristarship guard post's statistics? I'm envisioning that the OMD should just barely fail to oneshot a Mk. II zenith starship. (Or if that is too little, deal 2/3rds the damage of the HP of a Mk. III zenith starship). That should still be strong enough to severely threaten starships, still make golems sweat, but give people a chance to deal with them if they do accidentally bring big stuff into the system.
Either that, or keep OMDs as is, and reduce the spawn rate to just a little over superfort spawn rate (for the peacekeeper AI type, just a little over the rate that the Fortress Baron AI has for superforts), and jack up the price of the OMD from the trader and make the cap 1.