Pretty much every feature you listed is already possible by the player themselves. You can accurize/tune weapons for greater accuracy with gunsmith kits, you can already reinforce most clothing with various fabrics in the game with a tailor’s kit. You can add weapon mods for different barrels, ect.
An auto-fire weapon mod is technically in the game, but not listed to spawn anywhere or be able to be crafted so you would need to spawn it through debug. It adds a 3 round burst to pistols. I personally changed it to be able to be applied to most guns and threw it in an item group or two so it spawns naturally. I assume the reason it was made obsolete is because a one-size-fits-all trigger group doesn’t exist.
Gun modding in this game I feel is extremely underwhelming, but besides optics that’s the reality of most attachments and “upgrades”. At best, the most functional “upgrade” you will get to any gun part in real life is making it more durable. I could be wrong as I often am switching from gun to gun, but I don’t think guns really decay unless you use them as zombie smashers or use blackpowder loads. I for sure get where you’re coming from, I absolutely love customizing guns for special purposes, and spent a lot of time finding and upgrading weapons in STALKER mods (of which many were mechanical magic you couldn’t really do to weapons without industrial equipment). But in practical terms if you want to do this it would probably be much easier for you to create recipes with sufficiently high mechanics/fabrications skill + tool requirements to create the weapon mods you want.
All of that said, if you wanted to, you could make this into a mod. Where it really doesn’t matter too much how realistic or needed it is if you’d like. The NPC side of it I wouldn’t know what to tell you, but for gun mods you could easily add a lot of them just with a simple text editor and checking out existing weapon mods in the JSON files to see how they work, and then create a recipe for crafting them, or add them to item groups to spawn naturally.
Going the NPC route would be very complicated to implement from what I hear. It would probably be much easier for you to just add weapon mods you could either craft or find. Plus NPC’s are a WIP right now, so there’s no real guarantee any mod made this way would stay stable or work for very long for the extra effort it would require.