Going to Sanya via Shanghai

You need a visa. Sanya's visa waiver only applies to those directly arriving from and departing to third countries. In your case you will not be clearing immigration in Sanya so you will not be granted a visa waiver. Also 144-hour transit does not apply to those whose final destination is in Mainland China (The whole island of Hainan counts as a part of Mainland China politically).

TL;DR: you need a visa.