Here's a few of my recent sci-fi explorations. I think I'm getting better at this. Original resolution is 12k Still some room for improvement in several areas but pretty pleased with it.

I start with Stable Diffusion 3.5 Large to create a base image around 720p.
Then two further passes to refine details.

Then an up-scale to 1080p with Wan2.1.

Then two passes of Flux Dev at 1080p for refinement.

Then fix issues in photoshop.

Then upscale with Gigapixel using the diffusion Refefine model to 8k.

Then fix more issues with photoshop and adjust colors etc.

Then another upscale to 12k or so with Gigapixel High Fidelity.

Then final adjustments in photoshop.