I'm not very clever with Anki or Excel, but recently I realised that I have saved lots of words with rikai-chan that were already in the core6k premade deck, which I also learnt. It starts getting confusing, because I used core6k entirely with English meanings, however, my second self-made Rikai-chan deck contains only German meanings.

Is there a way of making two lists of both decks and delete the cards from one list of a deck and reimport it? Like this, I want to reduce also the review pile I have every day as I will be working fulltime from Octobre onwards.. Thanks for any help!

This seems to be a reccurent problem. I don't know wich version are you using of anki but in the 1.xxx version you can MERGE the 2 decks Based on one COLUMN for example the EXPRESSION field as you might know.
That is, would need to sacrifice statistics from one deck or the other if you want to merge them both into a single deck.

Otherwise if you need to keep both of your decks and eliminating from RIKAI deck, vocabulary that is contained in core6k your solution may be :

1.Add a tag (for exemple :CORE6K) for all your Cards in the CORE6K deck.
2.Export it as a CSV File.
3.Open your rikai deck, file=>import=>choose the exported file made in previous step.
4.Ok, now map your last Field to Tags.Other fields will not be needed for an update.
5.Click on Update, and choose wich fields correspond to the Expression Field.
6.ok, if everything is fine, you might find your RIKAI Decks with the new tag created in Step:1 (CORE6K).

There might be simpler solution I think but i'm still using this one.Make backups of your decks before trying in case my explanations turns out to be a deck.

Otherwise the problem can be found with keyword such as :"merging anki decks" etc.

What if you do this?
Assuming that the Japanese definition is the same for those two decks:
- you create a new empty deck
- the new deck has the following fields: Japanese word, English translation, sentence, ....
- import your core6k according to those fields (use File > Import)
<~ 6000> nodes added 0 deleted
- Let's call the Japanese word field: "Front"
- Let's call the English translation field: "Back"
- now let's import your rikai file (that may have Japanese word, English translation field). Make sure when you use File > Import, the japanese definition should be mapped to "Front"
and the English/German translation should be mapped the "Back".
Uncheck "Update existing notes when first field matches".

Now you have the original core6k list with the extra rikai-chan words.
Again:
- create a new deck
- import core6k, map the Japanese word field to Front
- import rikai file, map the Japanese word field to Front, do not allow updating already existing fields
- (You may do not call them "Front" and "Back", if you do not would like to.)
- You can tag rikai words with "rikai" using Excel, so later you can create a deck with rikai words minus core6k words


(I use Anki 2 btw.)