What should be done is referred to as selective pruning, and you should stagger the depth of the cuts. I could spend a lot of time explaining what that is but, of course, there are YouTube videos available to help me out! HereIsOne. (Not affiliated in any way--I just chose one and watched it to make sure it covered selective pruning fairly well.)

Once you open the shrub up a bit with the pruning, wait for the new growth to begin to make progress before going through to prune again. Hot weather can fry tender new growth, so you should leave more of the existing during these summer months to help protect it. In year-round growing climates, such as the many parts of CA that do not get frost warnings, you don't need to heed the warning in the video about avoiding late pruning so the new foliage won't get cold damage.

Thank you. have you actually pruned a Polygala fruticosa? I am just wondering if you can cut into bare wood and will it resprout?

wow. Yes, I have pruned polygala shrubs. However, my clairvoyance skills might be lacking. Without a photo, or even a description of the age and condition of your plant, it is impossible to for me to have any idea if your plant is at the end of its life cycle, diseased, healthy but overgrown, simply poorly maintained or growing under less-than-ideal conditions, judge how much growth you are planning to remove or much else of importance to answer your second question.

If you follow the pruning instructions, you will not see any of the prune cuts. Then, if the new growth forms in the interior, as would be typical for this type of pruning in the vast majority of cases--great. If for some reason it does not, due to conditions I have no way of ascertaining from the information made available, there was no harm done with the attempt.