You can find me over at https://thesearchengineoptimizationexpert.com/. I'm an entrepreneur with a passion for branding, SEO and startups. Although I really enjoy business related topics, SEO is by far my favorite. I've dedicated the last 10+ years to studying, testing and implementing SEO strategy.
It doesn't matter like it used to.. or in a traditional SEO sense like you may be thinking. The most important thing now for SEO is to make sure that your blog is being promoted well and that it's an active part of your social media promotions along with your website.
That being said, if it's on topic then I would put it in a directory. This will increase the overall authority of your domain as long as you actively link from your blog posts to relevant pages throughout your website on a consistent basis.
I deal with small business owners on a daily basis. Their knowledge of SEO varies greatly as does their competition in the SERPS. I will tell you that it is tough to give complete answers without first knowing how competitive your niche is.
If those 50 domains are to be of any use, you will need to promote each of them first. Setting up a site without any promotion just to link to your main site is a losing strategy and one that will leave you pulling your hair out.
It will not be helpful unless those sites are individually promoted first. That can be a monumental task, depending on many variables.
My deduction, without the minutia... It sounds like a better investment of your time may be to promote your main website first through other relevant media.
All of this depends on many factors. Would be glad to help you sort it all out on a call.