Short answers, pulled from the story.
Simeon I of Bulgaria became the first ruler to adopt the specific title tsar after his coronation in 913. His son Peter I continued using the designation throughout his reign until 927.
Stefan Dušan began styling himself Emperor of Serbs and Greeks in 1345 before his formal coronation. He was crowned in Skopje on Easter the 16th of April 1346 alongside the newly elevated Serbian patriarch.
The first Russian ruler formally crowned as tsar of all Russia was Ivan IV in 1547. Foreign ambassadors including Daniel Printz a Buchau in 1576 argued tsar should not translate as emperor.
Catherine the Great adopted the hellenized title tsaritsa of Tauric Chersonesos after annexing Crimea in 1783 instead of using tsaritsa of Crimea. By 1815 Russian emperor assumed title tsar of Poland following annexation of large Polish territories.
The term tsar entered English metaphorical usage since 1866 referring to U.S. President Andrew Johnson. It carried connotations of dictatorial powers fitting the official title autocrat held by Russian emperor.