I think the most important thing at any particular point in time is to use a break cue that you are confident using. That probably provides better results that night than any changes you might make. But if you are trying to improve something, here is what I think the relevant basics are:
0. Only two things matter: cue ball speed and repeatable accuracy. Accuracy is more important. The goal is to maximize cue ball speed (even if you don't use it all on a break) while keeping consistent accuracy.
1. Changes of a fraction of an ounce are in the noise. If you experiment with break cue weights, you can just use 1 oz. changes. Your body doesn't notice the difference between 18.6 and 18.7 oz cues, and the cue ball barely does.
2. If you have separate break and playing cues, there is no reason to keep the weights the same. The strokes are so different that the difference in weight has no ill effect. This the the same reason that while full-swing golf clubs are carefully weighed across a set, the putter is weighted without consideration of the other clubs in the bag. Of course, a weight difference is noticeable, and if it bothers you that they have difference weights, then don't do it. There's not that much to gain.
3. Increased cue weight at the same cue speed means higher cue ball speed. Within a range of 16-24 oz, a one-ounce change doesn't affect cue speed much for most male adults. (I don't know about females.) You can usually add weigh without losing any speed, or so much speed it eliminates the gain from the additional weight.
To see how you as an individual respond to different weights, use something like "Break Speed Pro", a $5 smartphone app (there may be better ones, or free ones that do the same thing). Start with no weight bolts in the cue, and add weight until cue ball speed drops by 1 mph. That's about the most efficient weight for your current break stroke.
With any given cue speed, you can increase cue ball speed by using a harder tip; phenolic is harder than any leather. I think any cue and shaft characteristics besides weight and tip can be ignored: use whatever you are confident using.