Is there such a thing like "18-months/half-time syndrome"?
Everything is possible: I'm pretty sure, from a large enough population of former PhD students, most will tell you that they felt demotivated at some point, but the timing will depend on the individual and the particular circumstances.

It is true, however, that mid-PhD corresponds to a particularly large number of negative factors, and it is common to feel bad about your thesis around that time. Heck, it's common even that there is a PHD Comics that highlights it:

      

(Don't mind the exaggerated x-axis scale. The area highlighted corresponds to mid-PhD.)

Now, why is that? Well, among your rants, most of the factors are actually listed in your question: Now, you know the field well enough to see not only the good, but also the bad in it. The initial elation has left, and you are left with the doubts. This is sometimes accompanied by deep questioning about your progress: Have I done enough? Have I taken the right course of action? etc.



But the most important point is: how to get out of it? Well, part of the problem is a natural “oscillation”, which means this is probably actually just a low point, a bad moment, and it will actually get better. Don't have too much fear of “spiraling down”: you've made it thus far, and you're aware of the issue!

As for more actionable advice, I would say:


Now that you are more knowledgeable of the field, you can actually start to make better choices: if you don't like a given approach, just steer away from it. You still have some time to do so, and it is part of your PhD to learn making strategic decisions (if you haven't already).
You may not see it, but you will be much more efficient during the second half of your PhD than the first, mostly because you have learnt a lot already and can make better decisions.
Pick a few challenges (one or two) that you would like to meet, and focus on those: you'll feel much better if, instead of chasing some holy grail, you can help solve these specific issues that you care about.
And remember: completing a PhD means becoming an expert in your field, and that actually means being able to critique its practices, recognize the good and the bad. It sounds like you have actually achieved this goal!


I hope this helps…