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.

Advice:

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.