I already have all other DLC, including Character Collection 1 and Character Collection 2 separately.
However, "Character Collections 1 + 2" still appears as a DLC I DON'T have in the shop - am I missing anything or do I actually already have all the contents from it because I own the individual Collection 1 and Collection 2 DLCs separately?

The same thing is happening to me.

1 & 2 pack lists the same content as buying 1 and 2 separately so there's no need to buy it, but it also means that if you bought them separately, steam will forever show that you don't bought that pack, just like it will forever show you didn't bought galactic edition even if you have all it's contents.

Can that trigger someone's OCD? Yeah. Does it actually mean you are missing something? No.

Check the DLC section in your steam client. Go to the game's page of your library, on the right hand side. I think it should list all the DLC individually there with a checkmark or not being greyed out or whatever.