SEO for a small self-hosted blog: how do I find a good SEO service, and how much should I pay?

I run a small niche blog with about 3k subscribers, Wordpress, self hosted. I’ve got about 500 blog posts since 2014. I don’t care that much about monetization, but I think viewership could be significantly improved with better SEO. Some of the text content is nsfw, but zero pornographic images.

I don’t have time to try and do the SEO myself. I’ve tried some, following some standard advice and guidelines, but haven’t seem much improvement. I want to pay an expert to do it.

1. How do I find a reputable expert? There are a zillion options and no way for me to judge quality. Should I look for an expert with experience in my niche, or is that irrelevant?

2. How much should I pay? One time, and then ongoing to maintain optimization?

3. How do I judge if my service provider is being successful? Increased traffic, I presume?

Thank you.