I don't know why anyone would think Ion cannons are rare. I just started and counted a few 100 system games.
None of the Mk I worlds adjacent to the Human HW had any Ion Cannons.
85% of other systems had at least 1 Ion Cannon.
65% of non-Core+ systems had 2 or more.
20% of non-Core+ systems had 3 or more.
Core and HW systems had at least 4, and as many as 6.
In all three games, there were NO OMDs on any non-Core, non-HW system. 1 each for those. Peacemaker aside, under normal circumstances, OMDs are only a concern for late game and really rare random systems, while Ion Cannons are very common.
I'm still thinking 12million per 20 seconds is about right. Dangerous in the early-mid game and there's enough other stuff on the Core/Homeworlds that the player probably can't make a bee-line for it meaning it stays alive long enough to hurt in the end-game.
12 million damage is enough to be noticable for low-mark Starships, yes. But it's so very little against Mk IV+, which will usually have 40,000,000 HP up to 128,000,000 for the Mk IV Zenith. That's not even 10% damage per 20 seconds. I'm far more frightened of the chance that a Mk IV Arachnid Guardpost will be within range of my entry wormhole - it'd be
15 times the damage for that same interval, and those spawn all over the place.
The OMD is a defensive Super Weapon and really should be treated as such.
Really, instead of gutting the damage again, why not remove the Radar Damping immunity? That makes it possible to use Raid Starships again, even on worlds with Tachyon Subcommanders. Maybe also get rid of the Nuke immunity, just to give a Last Resort counter - if you REALLY want that OMD gone, then EMP or Nuke it and pay through the nose in AIP for the convenience.
Also, fix Peacemaker to not get 2 per system - 1 is more than enough now.