When I started this thread there was no simple way to batch convert PNG files into JXL files, and the resulting conversions from the available converters (Converseen and Squoosh) were plagued with visual artifacts which ruined the image quality.
Some time ago I posted a link to a piece of software called Converter at The GNOME Way - #93 by Colin. Its aim is to convert image files into different formats. I decided to convert the wallpaper PNG images I'd previously downloaded into JXL files to see how it performed. I batch converted 58 PNG files into JXLs at 98% quality, and it took about a minute to do. The total file size of the images was reduced from 101MB to ~15MB.
I have uploaded pngs-jxls.tar.gz to Upload files for free - pngs-jxls.tar.gz - ufile.io. Its size is 116.1MB and it contains the 58 PNGs and their corresponding JXLs for the purposes of comparison. The only difference I can see when comparing PNG images to JXLs is the file size (and I'm wearing my glasses 
).
It would be great to see Garuda with a lot more wallpapers OOTB, and the creators now have a good tool to easily compress the image files so that this can be done.
If there are reasons why this can't be done then this thread can be closed.