I see that you localized "先輩" as "senior," but I don't feel that it's a perfect match, as in English the word senior has many connotations that 先輩 lacks, and vice-versa. For instance, my father is my senior, but I would not refer to him as 先輩.

Same here. Although 先輩 basically means "senior" but that's only in dictionaries. And we all know dictionaries are not that reliable translating things except for giving us the meaning we need for words which there are many not in any dictionaries.

Sempai or Senpai is fine to leave untranslated because we all know that word.