YOU SAID:
That time I got reincarnated as a slime is the best
INTO JAPANESE
その時私はスライムが最高なので生まれ変わりました
BACK INTO ENGLISH
At that time I was reborn because slime is the best
INTO JAPANESE
その時スライムが最高だったので生まれ変わりました
BACK INTO ENGLISH
I was reborn because slime was the best at that time
INTO JAPANESE
当時スライムが最高だったので生まれ変わりました
BACK INTO ENGLISH
I was reborn because slime was the best at that time
That didn't even make that much sense in English.