With "そうした方が", "方が" adds this feeling of your pointing to something specific in exclusion of others, which means there's another option being considered, and the speaker is preferring one over others.
"そうして" is an odd one out here. This only makes sense when she's saying it, because it is her that does something that makes her cuter, yet "可愛くなる" only makes sense when the observer is saying it. So the subject of two clauses do not match up. Perhaps you mean "そして、もっと可愛くなる". "そして" just means passing of time or unfolding of events, so in this case what's making her cuter is left vague, and what's getting communicated is just this sense of her getting cuter somehow.
"ら" in "そうしたら" is a particle that adds hypothetical, so the part that gets emphasized is that the speaker thinks there's something that makes her cuter, yet she hasn't done that yet.
"すれば" in "そうすれば" is also a form of する that adds hypothetical, so more or less the same meaning as "そうしたら". I spent good 5 minutes thinking about what differences in nuance these two forms have, but I can't really think of any. What I can say is that one is certainly not formal than the other.