Choice of project format can be tricky, as it does depend on what media you are using and what your final destination is. If you have multiple media types and destinations, then clearly you have to make a compromise of some sort. Rendering all your files to a common format will probably not gain you much speed, and you will likely have a loss in quality due to the extra render (not always true).

For example, I edit a mix of 4k UHD 24p camera footage and high resolution stills, with intended output 4k UHD or 1080p HD. My project setting therefore is 24p UHD.

You have a wider mix, but it seems the most common + destination = 720P 30 fps. So that should be your project setting.

Regarding:

What I'm looking for is perfect preview.

Well, don't get your hopes up too high. Vegas (like ALL NLE's) is setup to edit your footage, not simply play it back, and can give you a decent approximation of how the final render will look, but in my experience it is never "perfect." Playback performance is dependent upon many, many, factors. Vegas gives you tools to help you deal with this, including proxies and pre-renders, but these are more advanced workflows.

Yes, "ram preview", for instance, is such a nice tool. I guess I won't get anything better than that, as you say, too many factors. Maybe in 30 years computers will be powerful, latency free, and stable enough to deal with any video project in realtime / full quality.