While I don't need help in the technical sense I'd appreciate some feedback on what I am trying to do:
I am currently in the process of reinstalling and re-organising my Store content and would like to have a more organised control over what goes into what .dbc file. So I thought about whether it is possible to install, say, all the worlds first, take the respective .ebc out, install all premium content, take that ebc file out, too, and so on and then rename all .ebc files manually before putting them back together into DCCache. That way I would know that dcdb3.ebc, for example, contains only premium content, while dcdb4.ebc all CAS sets, and if I want to install more premium content I would simply take out all ebcs except for dcdb3.ebc.
Any ideas whether this would work?

To me, this seems a bit... packageceptiony.

Yes that is a made up word.

I think the best way of testing this involves not even loading the game, but the Launcher. Just use 2 store content of one category, 2 of another category. Open the launcher and see if all 4 packages show up :o.

I would be interesting to try, but I have no sims3packs, and a whopping total of 1, yes 1, store content item.

I'm not sure what "packageceptiony" is supposed to mean, but if you've got a lot of sims3pack there are certainly several advantages if you roughly know where the game installed each of them, especially when you want to get rid of problem files.

But anyway, I've begun to do what I said above and it does seem to work so far. My fear was that the file name of each .ebc file was referenced somewhere, which would cause the game to become confused when told to look for a file inside the ebcs, which were renamed manually by me and wouldn't match their original name anymore. But this doesn't seem to be the case and now makes me wonder whether I could perhaps even rename the file entirely from, let's say dcdb3.ebc to PremiumContentSets.ebc.

Interesting! I wonder if this would help with a problem I am having. I have noticed that after a certain point of installing content, my launcher will not open again. This seems to be a result of too many ebc files to open at once. Right now, if I want to install content, I have to reinstall everything all at once in one shot. Takes about 4 hours. But if I pulled out ebc files, maybe it would let me do them in batches like you. Maybe the launcher would actually launch. Hmm..

(I just need the launcher to install stuff. I bypass it to play the game.)

I'm sure that this will definitely help, because you will also avoid the creation of these .tmp files, which are nothing but duplicates of ebc files and start to crop up after perhaps 4 or 5 batches and slow the Launcher down tremendously.
You just have to keep one thing in mind: when you remove ebc or dbc files temporarily the Launcher will always start at dcdb0.ebc (or dcdb0.dbc) again. So if you want to install something into a particular dccache file you will have to rename that into dcdb0.ebc and have that as the only ebc file in your DCCache folder. Does that make sense?

Ah yes, and .ebc is for Store items .dbc for third-party content.

It's also a good idea to install in batches and avoid the creation of huge ebc files. Usually the Launcher stops at around 200MB and proceeds to create a fresh ebc after that, but sometimes the files get much bigger and this is perhaps not so good.

It works wonderfully, Babele! I had some store stuff that needed to be installed, but I was just letting them sit until I felt like dealing with them. (I love the Complete Your Set deals, but the installation part was killing me.) So many hours of my life now saved...thank-you!