Anyone switched to jxl yet?

Most of your post shows lack of understanding or even the attempt to research before stating your opinion. Lossy = loss = image data being discarded from the captured file. There are many techniques to discard image data.

jxl format is open source. it has nothing to do with dng containers. Recently dng containers started to use the jxl internally.

jxl is supported by darktable for a while. Go check the manual or even your export module.

jxl format supports lossless compression and lossy. It all depends on your settings.