I have not done that. I only use collections and sync them so i can use them as a "portfolio" to show my family.
Right now Lightroom is sync. 2300 images and that is mainly images that i have uploaded from my and my wifes phone directly to new lightroom. So there is not a catalog to migrate. Cant understand why those images always need to re-sync when i open lightroom on my computer. When it is like this i cant upload my new images from my camera online because it takes for ever.
You said the sync issue is with Lightroom (cloud) on your computer? Is that correct, because you also mention that you only use collections and sync those for your portfolio. This means that you are using Lightroom Classic. Are you using both Lightroom and Lightroom Classic on your computer? That could be causing your problem.