but we do know that there are many ships who were originally purely civilian vessels that have been militarized following the collapse. apparently that is often more cost effective than manufacturing new, purely military ships.
in addition to the small upgrades to various transports you mentioned, Gemini's flight deck was originally meant for small mining drones only
I am somewhat doubtful that the Gemini was originally a "purely civilian" vessel. It's armed roughly as well as the Mule is - arguably better, considering that the drone system means that you don't need to put PD in either of the medium ballistic mounts
and you get a flight deck on top of everything else. It has a shield efficiency of 0.8, which is typical for midline warships, whereas a shield efficiency of 1.0 is more typical for midline civilian ships. Unlike the Tarsus/Condor and the Buffalo/Buffalo Mk 2 conversions, you don't see any significant loss of cargo capacity relative to dedicated freighters. The only thing that the Gemini is missing to be a purpose-built light carrier is 1 burn speed. As is, it could easily be a purpose-built escort carrier or the space equivalent of the merchant aircraft carriers used in WWII, neither of which is remotely like a 'purely civilian' vessel.
Granted, some or all of that could very well change with the next patch, and it having just barely more OP than it needs for its armament in the current version suggests that it'll be unable to field the better fighter types without sacrificing at least some of its armament in the next patch, though that would still leave it looking like it's only as "purely civilian" as the Mule is.
its flux stats are awful by cruiser standards, despite the fact that it would've already needed to be able to operate the typical Mining Blaster in an original non-combat role -- which is part of why the other two non-missile mounts are typically only used for the very flux-light single Flaks. so i don't think it got any significant improvements in this aspect.
The
absolute values of the Venture's flux stats are low by cruiser standards. However, the Venture
also only has three weapon mounts that can actually generate any flux (unless using modded missiles), which means that it has the highest base flux capacity and second-highest base flux dissipation relative to its weapon mounts of any combat cruiser. On top of that, it has a tough enough hull and armor (the second-highest base hull and base armor values in the cruiser class, at that) that it doesn't really need to rely on its shield for much other than soaking high-damage HE shots such as Harpoons and torpedoes. Speaking of its shield, the Venture has a shield efficiency of 1.0, which is typical for low-tech warships whereas a shield efficiency of 1.2 is more common among low-tech civilian ships, and on top of that it's an omni shield, which means it's much easier to catch missiles/torpedoes with the Venture's shield than with, for example, the Dominator's, especially if trying to flicker the shield to avoid significant hard flux buildup.
Also, the primary reason why the two ballistic mounts are often used for flak cannons is because that's the only way to get 360-degree PD coverage on the Venture. However, the Venture doesn't really need full PD coverage since it can use its omni shield to catch missiles and since its armor is tough enough that you need at least a low-grade torpedo or several Harpoons hitting close to the same point to really start hurting it.
it still has the civilian sensor package, just like the Shepherd. and while military grade sensors don't seem to be worth the extra cost/complexity on ships originally designed for civilian functions, i doubt replacing/altering an already present military sensor package to a lower quality one would be worth the additional effort.
Regarding this and the Venture's power plant: One of the easiest ways to make a demilitarized or civilian version of a warship is to simply build it using civilian-grade hardware instead of military-grade hardware - which, incidentally, usually makes the demilitarized variant cheaper.
Also, I agree with Embolism that it makes very little sense for the shipyards to be producing military-grade Ventures which are then demilitarized by the operators, or which are demilitarized for sale to the current operators. With production being done by automated factories that work off of heavily-encrypted and possibly hardcoded blueprint chips, it makes much more sense to think that the shipyards are simply working off Venture blueprints that call for civ-spec components, or that the Venture blueprints simply call for older-generation mil-spec components which are comparable in performance to current-generation civ-spec components and have never been upgraded to make use of newer mil-spec components due to the Venture falling out of favor with or use in the Domain military for one reason or another (granted, if this is the case, it'd be more appropriate to call it obsolete than demilitarized, as calling it demilitarized suggests that it's been intentionally downgraded from the military standard).
I can totally see why the Mora exists in spite of the Venture: the Venture doesn't even compare to the Mora as a carrier, at all.
The Venture looks to me a lot like it's an early experiment in carrier design - perhaps contemporary with the Mora, or perhaps even older. It's very similar conceptually to some of the early carrier experiments in the real world, which include cruisers/battleships with flight decks built over some of the turrets, cruisers/battleships with half the turrets removed to fit a flight deck, more or less dedicated carriers that have 6" or 8" guns in casemates under the flight deck, and so on. Aesthetically, the Venture is closer to the Enforcer/Dominator/Onslaught than to the Conquest, which suggests that it's older than the Conquest, and we know that the Conquest slightly predates the popularization of fightercraft. This would suggest that the Venture design is old enough to be an early experiment in carrier design and tactics, one which fell out of favor in military service due to being less effective as a carrier than more dedicated carriers such as the Mora and less effective as a direct combatant than standard cruisers such as the Eagle or a hypothetical (but probably extant at some point in the setting's history) contemporary of the Dominator. Such a vessel would, however, have a certain appeal for civilian use in unsafe regions - it's a single, fairly tough ship which provides light cruiser level firepower for repelling pirates, has a reasonably capacious cargo hold, and has a flight deck which can be used to field fighters for increased protection or other light craft for mining or salvage operations.
"civilian" is just what you call something that isn't actually armed at the moment. nothing's stopping you from putting an M2 on a truck and making an infantry fighting vehicle: space edition
I would not refer to a truck with an M2 strapped to it as a military vehicle unless the truck was already a military-grade truck before you strapped the M2 to it. Militarized or armed civilian vehicle, sure, but not military.
Also, the difference between "civilian" and "military" is not the difference between "unarmed" and "armed." That
can be part of the difference, but there are many unarmed military vehicles which do not have direct civilian equivalents, and even when they do have direct civilian equivalents they're usually not identical; the military versions are usually more rugged and more resistant to damage to permit long-term operation with relatively little maintenance in a hazardous environment. There are also armed civilian vehicles which retain their civilian status despite being armed - for example, many merchant vessels in the Age of Sail (and no, armed merchant ships at that point in time were not usually structurally identical to contemporary warships; the warships typically had stronger hulls and framing to resist incoming fire and support the weight and recoil of their armaments, whereas merchant ships were typically more lightly built).