Many people use a single catalogue with many more photos than that, without slowdowns, and just think of the advantages of keeping control of all your photos in one place. For example, imagine you want to get all your best pictures with a certain keyword like "cat". While you are looking through the catalogue for the first half of 2024, then the second half of 2023.... the person with one catalogue has already created a smart collection for that keyword and rating greater than 2 and is enjoying their favourite cat pics.
Working with one big (synced) catalogue would be my recommendation, and you could bring all the half-year catalogues into it by using File > Import from Another Catalog. Archive those half-year catalogues.
But if you really want to continue to use them, and also sync all the photos, then you would again use File > Import from Another Catalog to bring them into the synced catalogue. You are then going to have to be careful about making adjustments or changing metadata, and where you make them:
If you make them in a mobile app, they will sync into the big synced catalogue - but not into the half year catalogues. What now?
If you make them in the big synced catalogue they sync to mobile but not into the half year catalogues
If you make them in a half-year catalogue, they don't go to the synced catalogue or mobile until you do something (repeat the Import from Another Catalog, or use xmp saving and reading), so making changes in the half year catalogues is making work for yourself.
Those are just a few of the complications and inefficiencies, and just obvious ones.
What I'd suggest is following my one catalogue recommendation. Import all those half year catalogues and see how well it works for you.
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