We have one 2019 iMac 5K here in our studio that we went ahead and did the update to Big Sur and Final Cut 10.5. We opened a copy of a recently finished 4K Library on it. The assets reside outside of Library on attached drives and represent a mix of various media types, 4.6K ProRes 4444, UHD ProRes 422, 1080p.mp4, .png, .mp3, .aif. files. The plugins used on the Projects were, ColorFinale, various MotionVFX transitions and titles, iZotope audio plugins, Twixtor, along with some of the built-in titles, color board, and audio compression.
We updated all the third party plugins before attempting this. Twixtor was the only plugin that crashed Final Cut Pro 10.5.
We tried uninstalling Twixtor and reinstalling it but it still crashed.
We then did a hard drive wipe and did a clean install of Big Sur and Final Cut Pro 10.5, along with all the third party plugins and other applications we use. That took close to two days to perform but it worked. Everything opened just fine including the clips that had Twixtor applied to them.