I’d say [name_f]Siana[/name_f] as see-AH-nah – it visually reminds me of [name_f]Liana[/name_f] which I know to be pronounced this way! And I think I like [name_f]Siana[/name_f] more than [name_f]Alessa[/name_f] for the nicknames, and because it sounds a bit fresher. But [name_f]Alessia[/name_f] would be a wonderful alternative!