I started playing with DragonflyDB to replace Redis as the app data storage for a Mastodon instance. Redis used 22 GB of memory, DragonflyDB uses 8.5 GB.

That's very impressive, probably in part because Mastodon uses a lot of sorted sets (for timelines) and Dragonfly stores those in a much more efficient data structure, resulting in a lot less overhead (see https://www.dragonflydb.io/blog/dragonfly-new-sorted-set).
Additional bonus: they provide a K8s operator, with built-in HA