Hi jomregan,
Congratulations on your admission to Darden! That's certainly cause for celebration.
There are really 2 questions at stake here, so I'll tackle them one at a time:
(1) Have you already communicated to Darden your intent to matriculate in a year and a half (such as by making a deposit)? If you have, then before we get to retaking the GMAT, you'll need to carefully consider the consequences of reneging on that decision. Besides potentially losing a deposit, there's also a reputational cost to going back on your word. While it's unlikely that you'd face any concrete consequences in the short-term, people do talk, and you take the risk of being perceived as somebody who will jump ship when a better offer comes along. That being said, if you haven't committed to Darden yet, then this isn't a concern.
(2) Retaking the GMAT is always a gamble, and you never know until after the fact if it will pay off or not. Increasing your score to 720-730 would put you squarely in the average range for all the schools that you mentioned, which is a plus. However, you would likely need to strengthen other areas of your application besides your GMAT alone -- assuming that you would apply in R1 or R2 2020 for matriculation in 2021, do you have reason to believe that you'll grow substantially in your job experience by that time? Is it likely that you'll be promoted between now and then, or that you'll work on bigger clients and projects? Will you have time to dedicate to writing new essays that are not just slightly above average, but far above average? Will you be able to convincingly discuss compelling new examples of your leadership abilities? The GMAT is just a baseline -- by itself, it's not enough to gain anyone admission to Kellogg, Booth, or Ross. You need to think about this as a holistic package. If the answer to those latter questions is "yes," and if you're prepared to really dig in over the next 9-10 months to knock not only the GMAT but also your work at Deloitte and your essays out of the park, then I'd say go for it! YOLO!