When I tried Apple Vision Pro I thought the quality might be good enough for coding. I don't think I could bear wearing it for long periods, though.

I'm using a Quest3 for coding when I'm travelling with the Immersed app. It is a bit of a setup, but it allows using a USB-C cable for the connection. It also can open multiple virtual screens. I usually have 4 screens open. The resolution is not as good as the VisionPro but having 4 screens in a slightly lower resolution allows me to be able to read everything quite well.

Do you find it good enough to consider using it as a second screen replacement in a normal office environment?

No. It's good for a couple of hours, but I much prefer my 27" screen at home for a whole workday. I haven't tried the same experience on the Vision Pro yet because that's not something Apple lets you try out when you book the free demo, so I can't compare, but I'd guess it will be similar because at some point you get tired from the weight on your face.

You will need to tweak things to work for your eyes and preferences. In the glasses field of view, black background = transparency; Solarized background in my editor works for me. Same with web pages, I used a browser extension to change white to something similar to solarized background in vscode. YMMV

I use the Xreal for coding while traveling. It's not as nice as a monitor, but it's definitely good enough for coding IMO.