Turret design suggestions:

It's definitely a problem that reaction wheels scale poorly, especially since our ships are more than large enough to provide all the reaction mass our turrets could ever need. Having reaction wheels remain an option is useful of course, we do need them for drones and missiles (which lack a multi-kiloton hull for reaction mass), but full-sized ships should be using gearboxes.

Sphere turrets definitely have a strong plus side to them though: when evenly distributed around a cylinder, they are completely rotation-agnostic. Because they can all aim straight up, it's easy to quickly achieve coverage about the whole cylinder. Basically, the only restriction on a sphere turret's firing arc is when the barrel hits the hull.

The main advantage of pillbox turrets would be that you can achieve a better depression angle (here defined as "pitching toward the hull") by making the box taller, without having to change any other dimensions. So you can achieve a bigger firing arc for a smaller gain in mass.

Though a rather simple way to achieve both advantages would be to just stick the sphere on top of a tower...

Electric motors for turrets rather than momentum wheels are a commonly requested feature, and I'll be adding them at some point in the future.

Well, let's look at a series of potential things the player could do, so we can narrow down what's potentially necessary and several solutions.

1: Motors: I assume we have somewhat "black box" motors as is which turn the momentum wheels. So the question would be, how much player decision is necessary here? Here are some of the options.

a) Player inputs Turret radius and power, and that spits out a rotation per second number. The player doesn't mess around with the materials of the rotating mechanism or motors at all. The computer just assigns some value for the black box that x amount of horsepower/watts requires y amount of additional mass in motors and support.

In this case, any turret would actually need two inputs of radius and power: one for the main transverse ring and motors, and a second motor and ring for the elevation.

This could bring up a second issue of how many degrees of moment the gun has in either direction. Mostly useful for the gun elevation, but could be important for others.

b) We actually include a motor creation tool, so your horsepower to turn is something actually calculated as a real machine, and not a black box. Not sure this would be a good option, but its an option.

2: Turret dimensions. If we move away from sheres, we now have length, width, and height to consider. This has two major options

a) The turret grows to fit the turret payload: Basically, you figure out how wide of a turret ring you want (which decides most of the width and height) and the amount of elevation you want the gun to have, and the program will give you a turret housing just large enough to hold the payload.

b) The player is given control over setting the dimensions of the turret housing, at least setting the length, height, and width. The player is then responsible for figuring out how to fit the equipment in the turret house.

This is potentially much more frustrating, but it will allow more options: for instance, once could actually decide whether they want the ammo stored in the hull, or they want to try and store them in the turret like some tanks. One can try to make the front more flat or pointy, wide or thin. do they want the turret ring to be the maximum diameter, or have the turret extend a good deal past that?

3: Armor. I think it would be nice to have a similar amount of freedom in creating armor as we have in ship creation. This could actually create an interesting change in some of the meta: on some ships the ship turret might be the single most armored point on the ship, rather than weak spots to blow a hole through the rest of the armor.

This, combined with b, could produce a potentially very deep rabbit hole to go down, and may add way too much complexity. I don't know. Finally,

4: Gun number: it has to be decided if 1 gun should be stuck with, or if you could put multiple guns in one. It would be especially interesting if you could put multiple different kinds of guns, such as a co-axial or a light point defence gun on the roof of a bigger gun.

So, these are just some things on turret design from a game design persective I could think of. I'm sure there's a hundred more.

I think turrets can be modules to themselves, rather like how we currently have launcher modules for missiles.

You pick a 'payload', in the case of the turret it would be a laser, coil gun, rail gun or chem gun.

Then you pick actuator type: electro-mechanical, pneumatic, or hydraulic. Electro-mechanical systems are the simplest: an electric motor drives a gear attached to the turret along your desired axis. It's lightweight, but it rapidly loses effectiveness as the mass of the payload increases and the torques needed get out of hand. Pneumatic systems use a pneumatically driven rack and pinion to drive the turret gear. They can generate more torque per input power, but they need a compressor - which means you use either an electric motor to drive it, or you have to divert some of your rocket's exhaust gasses. Hydraulic is just a beefier pneumatic system, that needs more power and can drive the big anti-ship turrets.
I'm not an expert on any of these systems, so maybe you don't need all three, but I thought I'd put it out there.

Finally there's an armoring section, identical to the ship one where you can design armor layers to cover it all up.

Well, the problem is "complexity" has a whole bunch of knock-on effects, on playability, functionality, implementation time, and even realism: the more options you give the player can in some cases decrease accuracy, as it creates more oportunities to push the model past the kind of things it is designed to handle and break it, or various assumptions come together in an unrealistic way, like our 150,000% efficiency rail guns. Or an option opens up the ability to do something without having to take in realistic issues.

There's an endless rabbit hole you can go down on turret design: as someone who play's war Thunder tanks, I've seen many of the minute ways you can vary design and how much impact that can have. And War Thunder Tanks is still a pretty simple model of a real tank's turret.

While there is certainly a huge amount of detail you can go into, it's not necessarily wise to do so: Keep it Simple Stupid still applies here. You want to do just enough, and not more.