The messy UI is more for the complication (is it necessary) vs. the look. The placement and look is not the problem for me.
The next question is how the implementation will take place. For example, would the slider add any sort of benefit? Is it necessary to mitigate that control, would any benefit happen on planet side?
As I have said before, imho, the benefit of dual queue is to be able to quickly switch back and forth from Social to Military production on a empire level. Switching a single planet can be done by editing a single queue. But when you have 100 planets, editing 100 queue is problematic. This is where the dual queue comes in handy with a Slider. If the military ratio is fixed, you lose that benefit, you cannot switch anything quickly, you would still need to edit stuff everywhere to put your empire in military mode.
Another alternative could be very smart and programmable governors that reprogramm your queues according to patterns you have setted up. But if you think a slider introduces complication, that would be even worse.
If we build ships in space and set the slider to only build military projects, it may not be immediately obvious why our buildings have stopped.
In GalCiv II we had those little icons to help you immediately see where your production was going. I think those should be in the Planet Screen of GalCiv 3 too. I cannot know what StarDock is planning exactly but I would not be surprise if they just have not implemented them yet.
The more levers one has to place gives a finer control, but it is also another lever to keep track of in the game.
I agree, 4x tend to be complicated games but a good design will try to minimize complexity without sacrificing flexibility. In GalCiv 2 we had 6 levers on the Empire level as well as planet focus. I used the Colonies List to quickly see where I had put a Production Focus, otherwise it was easy to forget where you had put specific planetary settings. I hope GalCiv 3 will allow the same in the colonies view ( currently Coming soon!)
I like the idea of a space station that has a fixed production rate and given bonuses / penalties determined by distance from the closest planet. Thus, the slider established is a fixed slider determined by placement on the map. If I want more production, then increase the production on the circle. It accomplishes the same effect.
With such a system you can never put 100% of your production to Military, it always depend on the fixed ratio. And the ratio is the same in peace time and war time. That is why I dont like it.