The Verdict: Need More Info
The citizenship shortcut for spouses needs marriage, so it opens to you only if you marry. Until then, residence in Croatia as a partner builds the four years you will need.
Article 10 of the Croatian Citizenship Act lets the spouse of a Croatian citizen with permanent residence naturalise without the eight years, the language test or giving up their current citizenship. It names marriage only, so an unmarried partnership does not count.
The Aliens Act is wider. A durable unmarried partner (izvanbračni drug) counts as a family member for temporary residence, and four continuous years of it lead to permanent residence under article 156(1)(1). A spouse's first permit needs no proof of means (article 65).
So the fastest sequence is residence in Croatia as a partner or spouse, kept continuous for four years, then permanent residence, then article 10 once you are married. Without marriage, the route is ordinary naturalisation after eight years.