[DnD 5e] Looking to create automated turrets as a security measure (Avishkar)

Edit: Fixed a typo

Edit 2: Players are level 2 atm. This is post is meant for when they are level 3-5

Hello all,

First of all, forgive me for the extensiveness of the post. I'm trying to give as much context as possible.

I'm pretty new to DMing, and I have very little experience statting things other than regular humanoid NPCs, so I could use some help with this.

I'm running a campaign on the Magic The Gathering plane of Avishkar (formerly known as Kaladesh), a plane of inventions and magitek.

Due to their story circumstances, the PCs are working for an organized crime syndicate at the moment, and I was thinking of statting up a couple types of automated turret types, to use as a security measure, just in case the PCs need to perform a heist or something. Guards can already be covered by the regular Animated Armor/Helmed Horror stat blocks.

As a plane, Avishkar has the following limitations:

• Magitek constructs are generally powered by a thing called æther, which is basically ambient magic that gets collected and refined into batteries and other uses.

• Regular (esp fire magic) is generally frowned upon, because it tends to interact explosively with æther based technology, but most spells can be re-skinned or re-flavored as some sort of aether device.

Given these limitations, I had these ideas:

• An æther-powered automatic crossbow controlled by a trapped imp/lesser demon in a gemstone, or just a Servo creature integrated into the machine.

• Basically the same as the above, but more powerful, so it would shoot beams or something.

• And finally, a truesight 360° "camera" kind of thing that sounds the alarm if intruders are detected. This could be a Servo with modified stats.

But yeah, basically I need to stat something that is a creature, but immobile, and behaves like a turret. I thought of using the Fallout ones, but I have no idea how to translate machine gun fire to autocrossbow, plus, +6 to hit, 120ft range, and 5d6 + 4 damage (Avg 21) per attack seems extremely excessive for a CR 3 creature.

Any help will be appreciated, thanks in advance.