Check https://nvanalyzer.com for a tool that allows combining Nessus scan files into logical scans, analyzing and comparing them.
Hi, just thought I would add my experience. We started life with Openvas and then added a Nessus Pro when our pen testers started giviing us reports from a Nessus scan. We were using Nessus pro for approx 6 months and exporting the results into Excel with resonable success. We then started to look at Security Center but were guided down the IO route as we were looking at a Siem solution to run alongisde. Tenable IO is a great product and easy to use, you can make API calls into the backend to export results into a SIEM, you can also do the normal export into Excel. The reporting in TIO is ok, I still think it needs work as a lot of the reports are fixed and it lacks the ability to create fully custom reports. I would say you would be best to sign up for a free trial to have a look through the TIO reporting options. Happy to take any futher questions if I can help.
It could be worth your time to take a look at VulnWhisperer (https://github.com/austin-taylor/VulnWhisperer)
It can integrate with various scanners like Nessus, OpenVAS and Qualys. So you can have multiple scanners and view the results in the same interface. The big use case I can see that you can purchase a certified scanner for your internet facing IP addresses while using OpenVAS for your internal environment.
Note that I have not personally tried it yet but I do know the underlying stack it uses (The Elastic Stack) and the reporting and data mining features are solid. Also I am in no way connected to that project, just find the approach to be good.