Sounds to me like you want ghostscript.
Windows, Mac, And Linux. + free
I have scripted it to sweep though file repos and lower storage space by hundreds of GB by just taking years of randomly encoded PDF files at various resolutions and shrinking them to normal viewing and printing sizes.
Marketing people have made 500Mb+ PDF files full of giant high res images, that I have shrank to 5-10Mb and put them side by side, challenging them to tell me which was thee and which I shrank.
Since you did mention JPEG2000, I do not think ghostscript supports that yet, have not used in a while so could be wrong. But you said ideal, not required.