The spirit and intent of your message is beautiful. However the min maxers will team up and craft end game gear for everyone on the team by day 3, so it has to be limited to looting only, so that online play is a long game just like playing solo is.
I play online in a small group, but even with the exp setting turned way up and doing nothing but hunting zombies for exp, we couldn't
possibly have a weapon-crafting skill maxed by day three. To say nothing of steel production (which can take multiple weeks to find a crucible and the skillbooks required to make the steel-tier tools/weapons)
and a solid supply of other miscellaneous crafting parts like glue, tape and mechanical/electrical junk.
I understand that you don't want to make it too easy to beeline to this stuff, but there are so many gates blocking the way already that I really feel like you're grossly overstating how easy it would be.
Also, if your only way to stop players from beelining towards something is to make those items completely uncrableable, then it feels like there's a design problem somewhere.
There's plenty of middle-ground between 'Trivially easy to create, possible by day three, if you minmax' and 'Game Mechanics literally forbid this. Totally uncraftable'
IMO, you're in that middleground already, but if you believe otherwise, then maybe something else could be done.
I dunno, off the top of my head, "crafting a purple-tier variant of an item requires a special resource that you can only get by scrapping other purples."
Some sort of 'Epic item-piece' that is in every purple-quality tool or weapon, so if you get a purple AK but you really want a purple shotgun instead, you can sacrifice the AK to use it's special purple part to (alongside all the standard parts required to make a shotgun) to make the weapon you
wanted instead.
You'd need to set things up so that someone with the skills to craft purple tier... shotguns can still choose to craft blue ones instead.
Otherwise it'd be possible to become so good at making shotguns that you can't make any anymore.
That's already kind of a problem though, but shotgun parts or whatever else is stopping you from making a high-quality one wouldn't be as rare as this theoretical purpleolium.
So... yeah. something broadly like "Purples are craftable, but only if you sacrifice another purple to make it" with a side-order of "putting perks into crafting skills determines the
maximum level of the item you can craft, but you can still choose to make shoddier ones, in case you lack resources'