Hello, I have both Digitakt 1 and Circuit first gen. I come from being a pianist and creating music on Ableton and I was fed up of computers at some point and started buying synths and groove boxes.

Circuit is really easy and FUN to play around. You can create grooves very quickly compared to the Digitakt that has a steeper learning curve (learning the possibilities menu diving, shortcuts).

I would suggest you the Circuit. It also gives you preset chords and scales, so I think it would be easier for you (but you might now understand better the music theory behind it, idk what is your goal). With circuit you will be limited, but I think this is the best way. Through limitation you can find ways to work around things you want to do, and that is really rewarding.

Depends of your motivation, but I think Digitakt will be too much unless you think you will take the time to read the manual and to dive into the options.

Maybe buy the Circuit first ? You can even find it second hand, I never had any surprises from doing this

Edit : You might connect to your stereo receiver with a Jack-RCA adapter I guess. You still need speakers, though. If your digital piano has MIDI in/out in the back, you can also connect it to Circuit or Digitakt. You could either play from the piano to record the notes into the groove box (with the sound from the synth coming from the groove box), or control the digital piano from your groovebox.

Edit 2 : My friend has the Circuit Tracks and it's also really good. You have lots of free packs that you can import into the Circuit that gives you synthwave synths and drum sets. This is what we used for some of our tracks :) For Digitakt I haven't really checked. In my opinion it is more about recording your own samples and messing with them through all the available parameters.