If you’re up for some more tinkering, and don’t mind running an “unsupported” environment, you could also try running Roon in a Docker on Synology.
I did this when starting my Roon trial 4 months ago on a comparable NAS (RS1821+ with 20GB RAM). This topic gave me a good starting point.
I don’t have too many AAC tracks, but never ran into any issues. YMMV.
Happy listening.