Curio is a popular all-purpose digital notebook application for Mac. Currently, it’s 30% off, so it’s for sale for $34.99 instead of $69.99 until Dec 28th.
If you’re interested in managing rich notes with an application that has a long history of success, you might want to give it a spin. There’s a free trial which may get you hooked. I tried to make use of such notebook apps myself in the past but couldn’t think of many use cases. I don’t plan complex projects which don’t fit into lists or Mind-Maps myself, though lots of people on the interwebs think Curio is terrific for brainstorming, planning, researching, or even task management. I like to separate these things, but maybe you don’t.