After fixing some issues, migrating my #matrix media off #matrix-media-repo and onto #synapse was surpisingly straightforward.
I've written a short step-by-step list on what to do and put it here for now:
gist.github.com/Kunsi/f9b68200…
Migrate Matrix media off matrix-media-repo into matrix-synapse
Migrate Matrix media off matrix-media-repo into matrix-synapse - 00-steps.mdGist