Yep, I'm sure cartels worth billions of dollars in annual revenue are producing tens of thousands of kilos using one-pot tweaker methods. Also which "readily-available" precursors are you referring to here? Can I pick up 10000 kg at Wal-Mart?
Non-professionally-manufactured firearms do exist, the aforementioned ATF traces indicate that they are far more prolific than Mexican imports. VICE produced a documentary almost 15 years ago on cartels in the Philippines manufacturing completely viable firearms without issue - in the woods.
Now you're making a different claim. You said that manufacturing fentanyl requires all sorts of specialized equipment and knowledge. It simply does not.
Does that mean cartels aren't sophisticated manufacturers? No, of course they are.
Second straw-man: no one said homemade firearms literally don't exist. The claim is that they are a rounding error.