The debate between UX and SEO has always been a pressing concern within the internet marketing community. While years ago these two factors were considered separate, as time passes the industry has realized that these two are not independent from one another but should work together.
That being said, I am always an advocate of launching a website as soon as it is ready. Of course this is only the case if all of the duplicate content, low-quality links and SEO black hat strategies have been removed. If any of these factors are present it can have a negative impact on site performance and where possible should be removed.
Like mentioned below, how long the website has been up can have an influence on ranking as well as other factors that you can be receiving credit for by not postponing the launch. In addition, SEO is a continuous effort that is never completely done, therefore I would recommend launching the website and then implementing your changes.