DR35 in the tech startup space is a decent asset. Backlink selling works but it is not the highest leverage use of what you have built.
A few options that fit better for a startups directory with analytics:
Sponsored placements in the directory itself. Startups pay to be featured, highlighted, or listed in premium categories. Make it a recurring subscription listing and you get monthly revenue instead of one-time.
Sponsored market reports. If your analytics data is good, B2B buyers like VCs, accelerators and analysts will pay for curated reports. Even a -299 PDF report beats most ad revenue at your DR level.
Newsletter sponsor slots. If you have any email list from the site, startup tool companies pay -1500 per placement depending on audience quality.
Job board integration. Startups need to hire. A simple job listing at -199 per post converts well if you have the right audience coming to a directory.
Backlinks would be my last option here. Rates are dropping and vetting buyers takes more time than it is worth compared to the alternatives above.