Do take a look at Korg Gadget - it is a tremendous piece of software. VERY easy to pick up and just jam even with the vanilla synths (Chiang Mai is one big sweet spot), but really also deep enough to produce pro level tracks. The additional KORG synths (especially the wave station and ODYSSEi) are high end emulations and basically indistinguishable from the original hardware.

One of my favorite OP-1/OP-Z artists just did a video with a whole bunch of iOS apps -- doesn't make it any more affordable when you price them all, but at least you can mix and match the parts you want: https://www.youtube.com/watch?v=d12SNIcO32Q

If you find it, let me know!

I have tried:

GarageBand - powerful, but hard to get into full flow. Frustrating lack of deep dive - you can tweak presets but not build patches from scratch.

Korg Gadget - full control over the synths. Pretty good piano-roll style editing, but hard to record live. Also, I think it’s something in the default reverb that gives everything a very contemporary sound... not great if you’re not a fan of that sound.