Hopefully I checked and I'm not subscribed to matrix:matrix.org via my local ISP matrix+element :). I noticed the archive json files have timestamps, so may be

Hopefully I checked and I'm not subscribed to matrix:matrix.org via my local ISP matrix+element :). I noticed the archive json files have timestamps, so may be 'archive since previous archive timestamp' would do the trick (if there's a way in the matrix protocol to do that, and if matrix software is efficient in ressources in this timestamp to timestamp case). And having a starting timestamp being channel join time minus a user specified number of days to go backward a bit in your archive.