I also really wanted to use libjxl, but abandoned it eventually. The encoder was horrible.
Also instead of OpenEXR we store our multispectral images as TIFF, with lossless lzw:2 compression. Lot of work with TIFF, but in the end much more flexible
Unrelated, I'd also be curious to know how their initial data transformation varies from that of your TIFF scheme.
With my tiff lzw:2 it only takes the diffs for 2 consecutive lines per each sub-image, which is a shame. But still 50% compression. With jxl it would compress down to 10% or more with a sub-image base.