Interesting recent finds:

• Google Zoom Enhance. “Using generative AI, Zoom Enhance intelligently fills in the gaps between pixels and predicts fine details, opening up more possibilities when it comes to framing and flexibility to focus on the most important part of your photo.”
• Nick St. Pierre writes, “I just upscaled an image in MJ by 4x, then used Topaz Photo AI to upscale that by another 6x. The final image is 682MP and 32000×21333 pixels large.”
• Here’s a thread of 10 Midjourney upsampling examples, including a direct comparison against Topaz.