In most cases, no.
The thing to understand about sub-niches is that while you’re subscribers originally found you by researching a particular topic, they’re probably also interested in related topics as well.
You yourself might be interested in search engine optimization, but you’re probably also interested in how to get traffic from YouTube, or paid traffic methods, or how to get people to share your page through Facebook.
Likewise someone who is interested in a sub-niche like “online dating” will probably also be interested in topics like “how to attract men”, “how to talk to men”, “how to develop confidence”.
When you attract subscribers through your website, you’re not attracting people who have one interest and one interest ONLY. You’re attracting a particular “kind” of person, and that “kind” of person is likely to be generally interested in the kinds of topics in your newsletter.
If you’re concerned, you might tweak the first few emails in your sequence to specifically tie the topic of the email to your subniche. But only if you are really concerned.
Otherwise just use the emails as they are. You can always tweak them later if you feel they aren’t performing well.