Not sure what the problem is. Can you verify plenty of free space on both system and Lacie drives?
Run Disk Utility First Aid on system and Lacie drives. To properly run it on the system drive requires booting in recovery mode: https://support.apple.com/guide/disk-utility/repair-a-disk-dskutl1040/mac
After that rebuild Spotlight indexes. FCPX uses this for expedited search. If damaged or corrupt it can cause problems: https://support.apple.com/en-us/HT201716
Make sure you have the latest version of Apple Pro Video Formats installed. Starting with Mojave that is in System Preferences>Software Update.
Reset FCPX preferences (step 11): https://support.apple.com/en-us/HT203477
After doing that make sure background rendering is off in FCPX>Preferences and all other prefs are as you expect.
Even though you have deleted the render files, try setting a specific location for cache. Select the library, then in the library inspector do Storage Locations>Modify Settings and under cache give a folder location. If it asks about including render files, state “don’t include”. This ensures all render files are rebuilt, including thumbnails and waveforms.
Then load project in FCPX, select all timeline clips with CMD+A and render the timeline with CTRL+R. Observe all render dots are gone. If you have any clips using optical flow or stabilization, make sure the analysis phase of those are complete before rendering the timeline. This helps isolate the problem between render phase vs encode phase.
If that completes with no errors try to export it: File>Share>Master File
Settings>Format>Computer
Video Codec: H.264 Faster Encode
Resolution: 1920×1080
Make sure the destination file is on a valid drive with sufficient free space and permissions. If any doubt export to /Documents or insert a thumb drive and export to that.
If that gets stuck or doesn’t work, make a snapshot of the timeline: select the project icon, right-click and pick duplicate as snapshot. Then open the snapshot, select all clips in timeline with CMD+A, then strip all effects with Edit>Remove Effects. Select all clips in timeline with CMD+A, then try to render with CTRL+R. If that is OK, then try to export (without effects – just a test).
If that works but the other steps failed, duplicate the timeline again, select 1/2 the clips and remove effects from those, try render then export. Based on these results try the other 1/2, then 1/4, then 1/8 and find where the problem region is.