I've read three new tips lately that may have an impact on this re-rendering problem for some people/projects. They helped me. Although many people may already be aware of these items, perhaps some readers will benefit.
1) The options for File>Delete Event Render Files and File>Delete Project Render Files each offer a secondary selection of Delete Unused Render Files Only. Choosing this option for all my of my Events, which can be done together, and each of my projects, which must be done one at a time, freed up more than a terabyte of disk space, and ended the eternal re-render problem on some of my projects.
2) Many sites and articles say you should never move FCP projects and events at the Finder level- that you should only manipulate files from within FCP. I followed that advice until recently. However, I have found other sites and courses that advocate some file maintenance using the Finder (when FCP isn't running). I've tried it, and it works for me. Especially if you have a project that immediately opens when you launch FCP, and either crashes the program or requires endless delays, consider quitting FCP, moving that project and event out of their respective folders, and into different folders, called something like Disabled FCP Events and Disabled FCP Projects. You will then be able to launch FCP without any interaction betweent the program and these events and projects. They will be inaccessible until you move them back into the standard Events and Projects folders, but moving them can let you work on other FCP projects, when one is blocking proper functionality.
3) Sometimes, simply having a large number of Events and/or Projects on the same drive seems to cause problems or make FCP slow. However, I prefer to keep all my video projects on my mirrored RAID drive, so that I am sure to have a backup. Using Disk Utility to create Sparse Disk Images on my RAID drive allowed me to sub-divide and manage my events and projects in smaller and more logical batches. The Sparse Disk Images don't waste any significant space on my drive, and they let me mount only the projects needed, so that FCP is running with fewer events and projects "open". Dividing my projects and events into four groups, and putting each of them onto a different sparse disk image, solved the re-rendering problem for several projects. It also makes FCP snappier.
Unlike option 2 above, this approach maintains fairly easy access to your projects. At any time while you are editing in FCP, you can double click on a disk image to mount it, and FCP will see it. You don't have to quit FCP first, although you can also mount the disk images before you launch FCP.
I had about twenty projects with the repeated re-rendering problem, and I think I am now down to three.