I should add that the thumbnail previews do not render in the background. The only thumbnails to render are the ones that I happen to be looking at on my screen. It's maddening.
I am trying to figure out if there's a way that I can speed up my workflow.
1. Shoot a timelapse sequence (let's say 500 frames)
2. Import as .dng with 1:1 preview & smart preview generation
3. Make edits to frame #1
4. Copy settings on frame #1
5. Paste settings to all frames
(2.) Why make 1:1 previews on import? They are only used when you zoom to 1:1 in the Library. If you don't create them at import, LR will make one for that specific shot if you zoom into it in the Library loupe view. There will be a few seconds delay while the preview is being created (whereas if it was created at import there will be almost no delay) but on the other hand you are not lumbering your system with hundreds of big previews that you don't need. Same question regarding Smart Previews. Why? They are used to stand in for the original shots only when the originals are not available, for instance because you are away from home and the storage drive. Do you really need them?
(3-5.) Select all the frames (Ctrl+A). Use the toggle switch on the Sync button to make it Auto Sync. Any edit you make to frame #1 will immediately be done to all the frames.
I Don't think the previews should have anything to do with the OP having random incorrect exports. Lightroom should rerender all the photos on export No matter whether the previews are correct.
The "Auto Sync" feature has changed my life. I didn't even know this existed. It's a bit peculiar to activate but once I figured it out I was thrilled!
I have found that when I paste settings to hundreds of frames that sometimes a few of them get ignored. I _just now_ was preparing to export a sequence from a collection. I was previewing it in Lightroom gallery before I exported it and, sure enough, found that four images right smack in the middle of the sequence were completely unmodified - none of my pasted develop settings had been applied! I always (before I discovered Auto Sync!) used to command-A the photos in the collection and paste develop settings but sometimes a few frames would get ignored. Very irritating.