It's an intriguing idea, and I love that you have a real life trial. Worth looking into, but there are three problems to overcome.
First is the false assumption that money and interest rates cause growth. It's actually the other way around. Humans prefer accumulation to feel safe. If I can't ever own my house, in your model, I can never feel safe. The money system is based on a basic human need, not the other way around, and. I don't see how you address that.
Second, your incentive to spend, to put the money back into the economy is the same with plus or minus interest rates. So you're not actually changing anything there.
Third, your system only works if the whole world adopts it, due to the current GDP standard. Otherwise your valuations will lag behind everyone else, resulting in a currency crash and zero cross-border exchange. Which is what you're doing now.
There is one more problem. We're experiencing inflation because interest rates remained low too long, spurring rampant spending and investment. By reducing into negative territory, what lever do you pull to reduce spending?