Instinct can come into this question, alongside your writing and publishing goals i.e. the instinct to take advantage of an existing market or to otherwise act on the availability of certain outlets.
For example, when I learnt that there were ways I could DIY publish my own books without going through the rigmarole of sending submissions, following all these guidelines, and waiting for the right answer, I knew I was onto something that made more sense to me and which I felt was empowering and fair. There were real-life outlets for making my book available and accessible to readers both on Amazon and Leeds Steampunk Market who were kind enough to let me book a stall.
Your personal values may come into how you decide to publish. Are you a person who likes to do things yourself, with freedom and control? Consider self-publishing. Are you someone who is looking for someone else to take the reins: consider traditional publishing – open-submission periods and literary agent submission. If you’re looking for a publisher to bounce ideas with and promote your book among other authors who publish similar books then consider small press.
But if you’re happy with your end product/final draft, barring some writing or editorial improvements you’d like to see, consider hiring a professional editor who’s suitable for your book. How you publish after hiring someone is up to you. Editorial professionals can help with both self-publishing and submission requirements. If you know what you’re looking for beforehand.