Will upgrade from a Synology DS124 to a DS224+ be worthy?

[Host-in-the-Shell]

I'll cut to the chase with you. Upgrading to a better NAS is most of the time, somewhat of a sidestep. NAS machines are built usually for a single purpose and often feature under-powered hardware (for a media server at least). Provided you avoid transcoding as much as possible, a NAS would be "okay" much like another ill advised hardware such as SBCs would also be "okay" but this is probably going to require that you convert over your media to the most compatible format for most client, which I believe is H.264 under a .mp4 container.

Ultimately, if you can't avoid transcoding, then you will have to contend with getting a better machine to act as your media server. Right now something that features an N100 CPU such as a Mini PC is recommended due to its great codec support and relatively low power consumption.