Bod Chen Po

Chéchentsa Tsédé of Gugé (1066) is the pick - the wiki reckons a couple of decades. Restart until you roll a Diplomacy education, respec into True Ruler, and vassalise your way to empire: almost everything in de jure Tibet can be swayed into your realm diplomatically except Maryül (claim it as dynasty head) and Mjinjaa - which, helpfully, is NOT needed for the empire title (subjugate it with high fame only if you fancy completeness). Create the de jure kingdoms above your targets to unlock more vassalisations, and prioritise gold for title creation. Then re-create the Empire of Tibet. The wiki's follow-on: vassalise into India afterwards and The Grandest Tour is right there.