Yian Garuga is 3, the Scarred version is 6 but the only notable thing it drops over normal version that I can tell is the one special mat you need for some augmenting.
You can absolutely farm Drachen full set without killing Behemoth unlike mentionned previously, there's a guide in the reddit thread, it mostly involves running Partbreaker and LBG, with a bunch of bombs. The setup is fairly easy to get but it can take a while(an hour or so?). I guess you might cut the times a bit if you build around the least annoying parts to get, the claws iirc are the most annoying ones since they have a shit ton of hp so it takes forever to break them(and the bombing hitzone is finnicky).
has a section on solo farming behemoth with all the sets and pictures of what to do, it's not that complicated.
However, I would also say it's a fair amount of time you're gonna invest to replace the whole thing a few hours in. It'll last you until Narcaruga basically but at that point it's just better to switch, and Narcaruga isn't like very very far into the game, there's only a handful of monsters before. Meanwhile you could just make a bone/metal armor basic shit gathering mats in Hoarfrost in the first Iceborne mission and get solid defenses to replace Defender or if you're feeling like you want to optimize a bit you can hunt some of the early T1 MR monsters that unlock instantly once you see the first monsters in Iceborne(don't even have to kill it), shit like Jagras and what not. So I'd say farming Drachen is questionable at this point. Without proper decorations Drachen isn't even going to be that good either.