Do not blame immich for running on shitty hardware. Just alone latency of the hard drives makes it slow as hell not even talking about a completely under powered CPU that is already at its limit to run a NAS and additional heavy services like Plex.
A SSD cache is a must for loading small files which is a must for a Photo cloud cause of thumbnails, previews etc.
Talking about features, maybe you should specify what you actually want and need instead of speaking only generally.