That Resonant Garden is over 1100 euros? Maybe the demo is bad, but I really don't see the point. I mean it's a box with a few microphone preamps, multi-FX units plus some pieces of guitar strings. I get that people want to support independent builders and creators, but there's a difference between supporting them and letting them take advantage of you. Get an Electro Lobotomy Sonic Forest and a couple of Zoom multieffect pedals instead :)
The funny thing is, that for the purposes of our particular milieu, standard synths meant for "regular" music are actually the unusual, out of the ordinary ones. Since we have to figure out ways to use them for purposes they weren't intended for. Having said that, I've tried a few drone/noise synths, they typically will sound pretty good (or at least you'll be able to squeeze some good sounds out of them), but they often end being one trick ponies. Nothing wrong with that as such, of course.
Btw, the UK's own Stylophone is bringing out their own, budget theremin soon.
https://stylophone.com/theremin-pre-order/