I would like to propose the following tag synonyms:
I've merged and synonymed the mentioned tags. I've also removed the improvement tag as it indeed seems to be superfluous.
I've kept the readbility tag though because it's marginally useful (it tells us that the asker is primarily looking for improvements in the area of readability as opposed to e.g. performance or robustness) and I also didn't want to bump 19 questions just to remove the tag.