In Vermintide 2: Winds of Magic, there was a system where you would unlock resources at the end of a map, and these could be used to unlock slots on weapons that you could then customise to your liking.
Take this system you already have, and add on to it a little bit.
In WoM, you would be able to put ANY modifier you wanted on the weapon from the beginning (crit, attack speed, etc). Remove the ability to do this, and tie these to unlockable achievements on EVERY weapon. So if you want to unlock the ability to get +% critical hit with a lasgun, you have to hit ~500 headshots with that lasgun. You then permanently unlock the ability to add +% critical hit when you modify it.
Do the same with most modifiers. If you want +% reload speed, you need to actually reload that weapon X amount of times. And so on, and so forth.
You then have one base version of every weapon that you can modify however you like. You can apply skins to it. Perhaps make more of the weapon skins tied to (easy to attain) achievements in game, like completing X missions with the weapon.
You can only unlock slots for each weapon with a certain amount of XP/resources, so you need to grind with the weapon to unlock the ability to have (for example) both +5% critical hit, and +30% reload speed.
It could be broken down into several sections for further customisation, so you might have 5 slots: each slot can be filled with something different, perhaps every slot is worth 2.5% critical hit/ 15% reload speed/ etc, so you’d need to dedicate the weapon solely to critical hits if you want +10% critical hit.
(For the record numbers do not matter and just serve as an example, I expect you to figure out what is fair).
This system is more or less available to you in the Vermintide 2 game. Just take it and build on it. This will provide players with the crafting system they want, where they can make the EXACT weapon they want. It will also give players a nice progression loop, where it will be a fair grind to unlock everything on one particular weapon, and it will take a very long time if you want to unlock everything for every weapon; but as you do unlock these things, your character will become more powerful.
This would fix all the bitching about RNG and shops and everything else too. Put it in the game and you will make a lot of people very happy, and you will have a lot of us grinding our favourite weapons to make them exactly what we want them to be.
And yes, some people will be pissed initially that they lose the perfect or near perfect weapon they lucked out on. But very few people have such weapons, and those that do will almost certainly be willing to grind out their favourite weapon again to unlock everything.