Hey there 
For individual use I have one namespace and a list for each “thing” I do - that’s usually short projects, my grocery list, “collection lists” for things like books I want to read etc. I usually dump everything in the list it belongs into and then add due dates and reminders, sometimes labels to make sure I actually do it.
Quick add magic is something I use a lot when creating new tasks.
For other things like software projects I create at least a dedicated list and work with labels, kanban and comments to organize everything that should be done. For bigger projects (like Vikunja itself) I have a dedicated namespace and a few lists - Vikunja for example has a list for Frontend, API, Marketing, etc.
Vikunja’s List are public: api | frontend
If you’re already using a task manager I’d start to use Vikunja like the old one and then gradually use more features of it.