There are multiple threads on this issue in this section.
The upshot of it all is that even though you will be classed as Resident in the ROC for the tax year, your employer could decide to be a total dick and continue taxing you at 18% for the first six months of the year.
The evidence you can offer to try and defeat your employer’s stance is located here: www.etax.nat.gov.tw
Go to the left side, click on 外籍人士稅務問題專欄, click on 外僑稅務QA, then choose the question fourth from the bottom: 所得稅法規定「中華民國境內居住之個人」 與「非中華民國境內居住之個人」如何區分? which should take you here: etax.nat.gov.tw/wSite/ct?xIt … Node=10767
That page says that in order to claim that you are resident in the ROC during a tax year, you must meet one of two criteria: either you are physically present for 183 days or more in a calendar year (impossible to prove before July), or that you are “domiciled” here, regardless of number of days you spend in the country. Since you will be registered on your wife’s hujitengben, you should be considered domiciled (the Chinese words are 有住所).