I think “reply as new topic” basically handles this! It creates a new topic for a related but separate discussion, linked to the topic it split off from. See:
But in general, yes, intended use / conversation style for Discourse is very different from e.g. Stack Overflow. As long as you’re more or less on topic, replies that continue the conversation are usually encouraged even if they don’t provide a single canonical answer to a question!
Another thing to keep in mind is that forum moderators can also split off any number of replies in a topic into a separate one, whenever they want. So if a sort-of-related reply ends up sparking a diverging thread of discussion that could stand alone, those replies can be batch-moved to a new topic later as needed.