totally agree that late afternoon/sunset is the time to visit Shwedagon given your time frame.

That is really THE highlight of Yangon. When I was there we also went to Sule Pagoda which is one of the other main tourist sights, but it's nothing exciting. My guide took me on the local ferry across the river to tour around and see more how the locals live which was kind of interesting. We walked some and took one of the bicycle rickshaws part of the time.

Frankly I didn't find Yangon all that interesting other than Shwedagon. The rest of the country has a lot more to offer. Especially Bagan.