Available here : advance-software.com/download/tools/gimp/
Source & win64 prebuilt.
Saves to j2k codestream format - bare bones as that’s all I need for now. Openjpeg supports writing a jp2 container around that to support to support colour profiles, etc. That can be added pretty easily. not by me as in crunch, sorry. Saves rgb(a) & mono(a) codestream.
Dynamic real-time quality tune, similar to jpeg save as that’s where the code came from.
j2k format & openjpeg have loads of tuning parameters that can be experimented with. Right now its super simple 0-100 which is effective but could be taken further.
No time to push a merge to GIMP trunk, but that’d be cool as it’s better than current GIMP j2k plugin, compatible license & ready to roll.
If anyone available to do this, please take it from here.
To avoid duplication of work, this might already be in process.
I dunno Gotta get on with other stuff. Have fun !