Demo trading is good for only one thing. Learning how your trading platform works. It lets you hit the wrong button at the wrong time so you can see the mistakes you might make when interacting with your trading software on live real account.
That's it, that's all there is.
The idea of a demo backtest is problematic, because you are trading the past. When you go live you will be trading the present and the future, not the past.
Start small in live, and scale up as you get better in identifying entries and exits. That's the holy grail. And you will find that only in real money accounts.