Is there anyway to compare the data to know exactly what is different between the two (like which has more metadata tagged on it or combine the tags)
Yes, you open the two catalogs and compare them yourself, visually. I am not aware of an automated solution.
Alternative: you can merge the two catalogs (or just the 2020 part of each catalog) into one catalog and then select the option to keep both sets of edits, which you can go through one-by-one and decide which to keep.
Additional comment: these problems would not happen if you use one catalog for all your photos. In general, I recommend one catalog for all your photos, the drawbacks are very few and the benefits are many.
Thanks, yeah I figured it might be a manual task, I've heard that once a catalog gets too bloated it will slow down, and I've definitely experienced that. Or do you mean back the files up differently versus transferring the files from catalog to catalog?
There are users with catalogs containing more than 1 million files who experience no performance issues. There are other users with catalogs containing 20-30k of files who complain that working in LrC is like wading through molasses.