Whatever last name you choose, I recommend looking up other well-known people who have the same last name. For instance, you may like the last name Thomas because of the poet Dylan Thomas or feminist Martha Thomas, but remember it's also the last name of fat homophobe Wendy's founder Dave Thomas and Supreme Court civil-rights crusher Clarence Thomas. You get the idea. If you're choosing a new last name, the suggestions about first names below also apply. While I don't care which one they use, I prefer the first one.
Vowels are the usual culprit for alternate pronunciation.
If you go by Cindy but insist on spelling it SynnDee, you can pretty much count on it getting misspelled every time. Our fraternity cook spelled her name Joan, which she pronounced Jo-Anne. If it's gonna bug you that people mispronounce your name, you might consider a name that is pronounced just one way. Sometimes an idiosyncratic spelling can be fun or interesting, but it could add to pronunciation confusion or look pretentious or ridiculous. Umlauts, accents, etc.