I agree
i also feel like it should be vulnerable to thunder damage. 

Yes, because thunder damage is concussive sound. I'm adding that to mine.

Edit: If you want to give them a magic item, Boots of Striding and Springing work great, especially if you build the arena for it - they can jump between hard-to-reach places while pelting the players with arrows, but go down to a few good ranged attacks. Giving them Gloves of Missile Snaring and swapping their shortbow for a hand crossbow as well makes them a CR 1/2 in my opinion, as ranged attacks become rather ineffective and the skeleton might be able to use Vex as well.

My fav creature

stealing this for a dungeon crawl btw (hp low key should be 14 or move a bit faster lol)

I don't care if it's suboptimal, I want a skeleton buddy and Pact of the Chain is my new life now.

Also, the art is good. I love that shortbow dang, nobody's ever made a shortbow look cool before.

I nerfed the skeletons in one of my dungeons, took away their bows because one of my players is a Minecraft player. Otherwise, I love customizing their weapons. I mean, not every person gets buried with a shortsword and a shortbow? Unless it's a local custom...

As a Necromancer, would it make more sense to use Skeletons or Zombies? Aside from Undead Fortitude, Zombies seem kind of just worse than the skeletons, with less range, much lower AC, as well as being outsped and outranged by Skeletons, as well as worse saves aside from Con and more notably Strength.