By the way, I just noticed that Sigil offers reports about the epub.
Among them, one is "Characters in the files"; in my case:
Code:
!"#&'()*+,-./0123456789:;<=>?ABCDEFGHIJKLMNOPQRSTUVWXYZ[]abcdefghijklmnopqrstuvwxyz~*§¬°ÀÇÉÊàâäçèéêëîïôöùûüœσ—‘’…→↔↵∀∃∧∨∪≡≤
This makes it easy to select which characters to include when trimming a font.