Which Synth to begin ? And choose the right keyboard ...

[Kaffimusic]

You say you want to learn how to handle synthesizers, but then you talk about a Nord piano. This is no synthesizer. There are the Nord synthesizers like the Lead A1, there is the Nord Stage has has a synthesizer section (but not very powerful, just enough to get the job done) and there is the WAVE series that is basically a sample player with a synthesizer section, that is practially the same like in the Stage.
When you really want to learn how to use a synthesizer, I´d either recommend a A1 (which is a so called VA= Virtual analog synth), it´s pretty easy to learn and use, but the keybed is not really good. When you want a big keybed, there is the Stage series, but you buy all the other stuff with it that you might not need. You can buy a rack/desktop synthesizer with hands on controls from any other brand as well and connect it to your home piano (as long as it has Midi Out). There are very cheap and very expensive ones, depending on your preferences. Advantage: This thing lasts and is useful, even when you change your main keyboard.
You´d need a synth with a fully knobbed surface, with one knob-one function layout. Good to learn the basics. But this type of synthesizer is usually not built with 88key weighted keybeds. So there are desktop models, supposed to be connected with your keyboard.
The Roland JX-08 is a nice polyphonic VA synth with a decent sound for around 350,-EUR. It´s small and can sit on your piano. Might be fun to use and learn, it has all the controls a (Virtual) analog synth needs.
You can spend 2000-3000,-EUR (and more) as well for a real analog synth like a Sequential OB6, Trigon, or any other "real" analog synth as well.
But the differences in sound just might start to make sense for you once you have heard and tried enough other non analog gear.
Watch youtube videos about different synth, and you might get a clue what to expect.