I have a Synology NAS with 4x 12 TB Seagate IronWolf Pro drives (one drive as redundancy), as well as 2x 500 GB WD Red SN700 SSD caches. I also upgraded the RAM from 4 to 12 GB, so it can run more services in parallel without chocking. The total cost was about 1500€ for a little less than 36 TB usable disk space and imo very nice performance.
I also make sure to get reasonable file sizes, usually around 8-12 GB per 1080p movie. I don’t really see or hear a difference with bigger files, as long as they have the same video resolution and audio channels+bitrate.