How do I find middle ground between content and SEO as a copywriter?

You can strengthen SEO on a blog post or article without compromising the content very simply.

1) Article URL - Use Keyword Phrase
2) Article Page Title - Keyword Phrase
3) Article h1 tag - Keyword phrase
4) Keyword Density - between 3-5%
5) Internal linking from other posts
6) Keyword Anchor Text from other articles - Use Keyword Phrase
7) Sharing link on Social Media to relevant groups
8) Configure image URLs with keyword Phrase
9) Configure Image Alt text - Use Keyword Phrase
10) Sharing link on social media on wall/post update

As you can see, with the exception of (sort of) keyword density, none of these really have anything to do with the quality of the written content.