I think I found a solution!! The latest version of Lightroom Classic doesn't sync keywords even once anymore, and I tried everything I could think of to work around this huge problem. After all, what is the use of having 50,000 images at your fingertips if you can't find what you're looking for? Try this: Open Lightroom Classic. Open LR on your desktop as well. Physically drag and drop images from Classic to LR. Voila! All your keywords will be there. No, they don't sync, but that's ok with me. Good luck!!
By @ScanVT
The problem with that approach is that when using "drag and drop" into LrDesktop you are actually doing an Import, which means that you are putting originals into the cloud rather than the Smart Previews which LrC adds to the cloud when syncing directly. Consequently, each image that you add by "drag and drop" (or by standard Import) is then using up cloud storage space, and if you do that with all 50K images you're likely to need over 1TB of cloud space.
Also, you would still need to save metadata to XMP in LrC before importing into LrD in order to get the LrC-added keywords into the cloud.
And one more complication: if the image that you drag and drop into LrD from LrC is not already synced as a Smart Preview, when the "import" into LrD is complete a copy of that newly-added original is then downloaded back into LrC where it will appear as a new image in whatever download folder you have specified. So that would be in addition to the original image in LrC.
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