@hafeatherston
This is what I would do. I have had to do this many times. An important point to note here is that my approach is based on the idea that all my original images are stored locally, on my Mac or external disks, I currently do not rely on Lr Cloud to store any of my originals. Any files that I have synced with Lr Cloud are there for viewing on my mobile devices and occasionally editing.
Incidentally,
I do allow Lr on my iPhone and iPad to download images I have taken using the camera app within the Lr apps but once they have downloaded I tend to move them to a folder on the local disk just as I would had I taken them with my DSLR. I do not store\keep any images in the location as specified in preference.
I do not allow the Lr apps to synchronise with my iPhone or iPad camera roll as I then found I had duplicates with a copy in my iPhone camera roll and in Lr. This messed with my head!
The steps
Make sure all the originals of my images are in my LrC catalog. I would make sure I have good backups of my Catalog AND all the images referenced from the catalog.
Untick all Collections so they are no longer synchronised with Lr Cloud. Allow some time for the sync to process.
Select all the images in 'All Synced Photographs' and Remove them. This will have separated the images in your Catalog from the Lr Cloud.
Open Lr (desktop, web, mobile) and remove any images, it might be necessary to check all three of Desktop, Web and Mobile
At this point you will have all your images locally stored and securely backed up with
no images stored or synced with the Lr Cloud.
Exit LrC, allow another Catalog backup to be created and make sure the LrC Catalog Optimises.
Open LrC. All Synced Photos should be showing zero.
Now start again. Now you can turn on any Collections in your LrC Catalog that you want to sync with the Lr Cloud, they should all sync properly.
I cannot stress enough how important it is to have a backup of you Catalog before starting - just in case.
I hope this helps.