I used to make compost similar to you. But later discovered that keeping compost aerobic is key. The problem with using a black plastic bag, there is a risk that compost can become anaerobic. Which doesn't promote beneficial microbes.

I used principles from soil food web. Have you heard of her, soilfoodweb.com 
She is very good. And there are many yt videos showing the approach. I've managed to make complete compost in 8 weeks! At the time I thought this was impossible.