Can you try adding cma=128M to the end of /flash/cmdline.txt (with a space between "quiet" and "cma...")? You can also try with cma=64M.
This, in combination with gpu_mem=192 allowed me to view the 60Mpix file on a RPi4 with 1GB total_mem.
Software decode seems to work fine, but it needs quite a lot of memory (I saw kodi jumping up to almost 400MB resident size during image loading (but then it quickly dropped back to saner levels).
so long,
Hias