The easiest DMA controller to use for mem2mem stuff on H7 is MDMA. The other dma controllers can’t talk to DTCM and ITCM. MDMA also has a truly epic feature that you can make a linked list of transfers in memory and it will go through them all with only one intervention by the program.
I’ve actually implemented a DMA caching scheme on a PS3 SPU (for a reverb no less) very similar in concept though somewhat more sophisticated to what I mentioned.
Granular should be a dream setup for doing DMA caching since you dont usually change playback speed or direction for the entirety of a grain so prediction should be simple and accurate.