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 group is created below with the render as the base and empty instructions, and 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.