The only reason to pay the premium for NI products is if you're deeply in love with their ecosystem. The biggest thing I see here is that you didn't mention Native Instruments at all in your reasoning. The action of the S88 is very stiff and heavy. Doing it on a sub par action can not only lead to injury, but it's actually just harder to do. It sounds like mumbo jumbo and fancy features, but piano playing is a very acrobatic instrument in terms of hand movement. If you're keen on learning to play the piano, get a digital piano with USB midi out and a properly graded and weighted hammer action. Oh and as others have pointed out, the s88 is the only one with fully weighted keys, resmbling a piano closer than the semi-weighted keys which to me, feeling wise sits in between synthesizer and a piano. If you are not interested in the native software suite, there are better value for money midi keyboards/electric pianos in a smaller form factor. The integration works well with ableton in my case. For most people the included packs will have everything to get serious, plus provides an upgrade path by adding more native products like Komplete down the line. I think the strongest case to make for the Native Kontrol series is the integration with the native software and the wide range of plugins and sounds you get from the start. I'm more comfortable with the interface on my desktop, so I don't use the screens or knobs much on the s61.

For house/techno production I usually just play chords or leads with one hand so I realized the s49 would actually be enough for me. Playing classical music you really miss the extra octave because you play both handed and the range can be very broad sometimes. I played classical piano for 10+ years but switched a couple of years ago to house/techno production in the box so can approach it from both angles.

Native instruments s88 mk3