the sultans of Ottoman empire still had the title of kayser-i rûm (Caesar of Rome) until the _sixteenth century_.
A bit over 840 years before the first Muscovy Tsar of Russia.
The Ottoman sultan just took it for himself because he conquered the territory and the people living there.
It was a big highlight of my adult life to learn that each country pronounces latin their own way cause they map their spelling over it.
E.g. Italians read the "c" in Caesar as the ch in 'chair", Hungarians as ts in tsar, Americans (anglophones ?) as s in sea.
But yeah the German pronunciation of kaiser is probably closer to classic latin.