How do I keep working on a render?

Use it as the base of the next one. In Canvas AI and AI Chat, press “Use as base” on the result: in Canvas AI a new engine appears to its right with that render connected as its base, plus an empty instructions node; in the chat the render becomes the base for whatever you ask next. In Nodes, connect the Generator's output to the image input of another node: another Generator, Enhance render, the Upscaler…

That way you iterate in layers: first the composition, then the materials and finally the light.