Redirect to my main Blog

I researched a lot and found a domain with very high-quality backlinks (niche relevant) and high domain authority. The price was very cheap in contrary to the others I have found that were in the 1,000s and purchased it (not at an auction, from GoDaddy) and redirected it to the main page of my blog. Lately, I have lost some rank and although my blog is earning good money I think that this will stop because of the rank drop in some KWs. Do you think that this is a good practice or do you think I should be using it as a separate blog?

PS. It was also a blog similar to mine and the domain age is over 12 years old.

Anyone? Also, should I redirect the domain to the first page of my blog or to another page I am trying to rank?

I don't see how simply redirecting a domain to your current blog is affecting your SEO ranking negatively.

If you are providing quality content, and you are using Google's best practices, I don't see how it can be affecting the ranking.

Let's keep in mind that Search engine optimization is a bit trivial as well. There is certain things you just can't control regardless because Google's algorithm will never be revealed so we really don't know exactly what is going on underhood. We can test things. We can read Google's best practices and guidelines. We can speculate based on past experiences. But we can never be certain.