I don't see the artillery starship as being the one-neuter-all strategy with the current guard posts that are completely immune to antimatter bombs AND have radar dampening--raid starships or groups of bombers would still serve the main purpose of sniping off posts. I think this is fair and perhaps the guardians need to be made immune to antimatter bombs too (unless they are already), but let it snipe off starships and stuff like forcefields.
The AI would have access to it as well; give it antimatter bomb immunity too to let it snipe off player starships without it being immediately overcome by the player's own so it forces the player to elicit a response.
Then are we not back at essentially the "Antimatter Starship" we had a short time ago? I am of the opinion that a long range artillery unit will either dominate because its range is so much longer then the other units in the game, or it will be almost useless because so many units are made immune to it in the name of balance.
Perhaps it would cost resources to provide the artillery starship with munition/ I'm thinking a similar Build-Point system as the Hive Golem or Hybrid equipment, yet instead of like the Hive golem building ships inside of it, have a certain number of points create a shell in the Starship. Every time it fires it subtracts 1 from that pool of shells it has (Just an integer, not actual shell entities). While the starship was actively accumulating build points and "building" shells for its use, it would drain directly from the player's metal and crystal Make it to where they can only "Re-arm" on allied planets and we have a ship that is both effective in its long-range role, isn't always the most cost-effective option, and has to be carefully used in regards to what targets you select for the ship to strike at.
In regards to it's ammmunition capacity, I'd say mabbe 20 + 10*mk, so 30/40/50/60 for how many shells each respective ship can hold. Perhaps the mark IV could have the ability to rearm itself anywhere the player has supply and not just on allied worlds.
It may be something that is a bit beyond the scope o fthe game in terms of micromanagement, but maybe someone can adapt this idea to a more workable outcome.
EDIT: Sorry for all the typos; new keyboard, getting used to the vastly different spacing of the keys
Hmmmm, don't think I've seen this idea before. However at first glance I'm not sure. I could see that on a specialty ship or maybe added to the artillery golem, but we are discussing a core ship here that a player will have access to every game.
I just took a look at the numbers (5.027) and the Plasma Siege has the longest range of starships at 10,000, the next longest range is the Fleet line at 6.250. (Goes up a bit with Mark Level.) That means the current plasma siege fufills its role as the long range bombardment starship to fly with your missile frigates and other long range units.
Running the numbers in a spreadsheet, the dps looks high enough so I don't think I would change that.
The only change I would make is a small increase in range, to maybe 11,000 to give a bit more survivability, but it's already at 5million HP for MK I so I'm not sure it needs that.
TL;DR: I think the current plasma siege is in a pretty good position and don't want to see major changes at this point, it fills its role of long range, without being artillery range, starship. If you want an artillery unit in your game, grab one of the bonus ships that are 'artillery', that is where something like that should be, not a core unit you have access to every game.