There were issues along the border with Russia.  After a couple of impressive Manchu military campaigns to the fort at Albazin, both sides in 1689, met at Nerchinsk, a Siberian outpost.  The Treaty of Nerchinsk settled the border between China and Russia.   It was the first treaty between a Far East Asian State and a European State.

While the Emperor was tolerant, there were limits.  Those limits were exposed in the Rites Controversy.  Pope Clement XI in 1704, banned some of the rituals practiced by Chinese Catholics. Kangxi was enraged and issued his own edict banning Christianity in China and Christian missions. 

Issues with his many sons all seeking to be designated as the crown prince plagued his reign.   In 1721, Kangxi fell ill during a deer hunting trip.  On his death bed about a year later he assembled seven of his non-disgraced sons and told them which son was to succeed him.  Emperor Kangxi epitomized the Mandate of Heaven. A true Renaissance Emperor and man.