Many beginners oversee that the perceived sound of a theremin does depend less from the setup and settings, and most from the playing technique, especially from how the right hand does transitions between notes (aerial fingering) and how the left hand is used to shape musical phrases without „pumping“ each single note.
Many years ago, there was a video snippet on YouTube where Lydia Kavina played a simple melody on a Gakken Mini Theremin. The Gakken Mini was a very cheap and crappy DIY pitch-only theremin kit, sold together with a Japanese electronics magazine and it sounded horrible (small integrated speaker overdriven by distorted square waves) in the hands of most people. But as soon as Lydia played it, one forgot immediately about that and one heard Lydia‘s typical expressive playing style (even without volume control!) which made forget all the above constraints.
That proves the teaching of professor Martenot (inventor of the famous „Ondes Martenot“) who said to his students about his instrument : „In front of you, there is a box with electronic components and controls which produces only cold and unnatural sounds by itself. YOU are the musical instrument which modulates these into music!“
From which I deduce (and that confirms my own experience) that no setup can replace years long practicing and tuition by an experienced teacher. Nowadays, the internet can be of great help with tutorials and instruction videos. But nothing can replace one to one lessons at least from time to time, where an experienced teacher analyzes the weak points in your stance and playing technique which may limit or block your progress at some point and indicates individually adapted exercises to guide you towards better results with less effort.
Your first instrument should not be too simple or too cheap, so that you don‘t feel limited or get frustrated. The Moog Etherwave Standard or Plus, or the recent New Etherwave are well suited from the beginning to a very advanced level. And they haven‘t all the whistles and bells which could distract the player. They are honest because they don‘t mask anything and force them into serious work.