Oh man this is one hell of an idea, compared to a lot of biome ideas I've seen this is probably one of the more fitting ones for the game itself. Vilethorn is one of my favorite evil biome tier weapons too, so that makes me enjoy this biome even more to give it a better home. The dungeon theory alone got me to believe in its viability, but one issue I do have is that the entire theme and coloration of the biome will be very very similar to the already existing Jungle. It'll feel very redundant making new grassy and green weapons and armor that the Jungle already has done primarily. The Behe-Moth boss is a great pun I enjoy but is one of the things that feels way more...Jungle-y to me. You mentioned how inspiration came from the Cordyceps, so why not give a bit more fungus in the design and maybe make it a lot more swampy?
Now for the armor from this biome, I know the difference between Crimson and Corruption loot can be pretty different but when it comes to their armor sets, they are primarily defensive melee class oriented. You stayed true to that, but I'd say the thorn aura is a bit too out there as a unique idea, it feels unfitting when paired along with the Crimtane and Demonite armors. That's just my opinion on how it fits in, though. Otherwise I do really like the idea of the thorn aura, maybe I see it more viable as an accessory from a Virus Kernel drop.
The raw damage buffs confuse me a bit, does "raw" damage mean just any damage or affecting a specific part of the game? With that aside, if it's any damage, the Ring of Force seems kind of crazy. Now for Miasma...I like the concept of it and the purple+green color theme would look nice (Though note the Corruption sorta did that, too.) but it seems really confusing how the buff works. Cursed Flames is a buffed up On Fire! and Ichor is generically a defense debuff, but Miasma has three steps of effects? Alike the whole Thorn Armor thing, this just doesn't fit in with the Crimson and Corruption. Also the Curtain-Ripper is nice but seems a little underwhelming. I guess compared to the Corruption vs Crimson, it's at least on par with the Scourge...
Like I said I enjoy the idea, and thats why I'd wanna see it become more possible as a reality, thats why I felt kinda like getting a bit critical about it. Who knows, I might be very wrong about a lot of this nor others may have mentioned it before me. So yeah summed up, great idea but a bit too similar in theme to the Jungle eventually and needs to fit more with its other buddies, the Crimson and Corruption.