Synology 10GB ethernet with 4k UHD transcription

I’ve been waiting for years to purchase a NAS so that I could ensure that all of my personal files are stored in a safe place and not on my 2 desktop HDDs. Now I am ready to purchase and need some feedback on what my best option(s) would be.
Use is for small business/home use. Mostly media streaming in and away from home. I will be digitizing all 800+ DVDs, home movies and pictures so need to ensure that no data is lost if a drive fails and it’s easy to hot–swap failed drive yet not tax the CPU when writing. Raid 1 or 5? HDD vs SSD?

My router is a Netgear Nighthawk X10 with 10GB SFP port available

Requirements:
4k Transcoding
Able to run Plex server with at least 8 simultaneous connections
single 10gb connection
H.264 & H.265 capabilities
IP Security System storage and playback (if possible)
HDMI output for connecting monitor for security system (if possible)
Mobile/IOS friendly
Windows-based . realistically less than $2500 but would consider more expensive options if worth it

If those 8 connections will be streaming content in the local network then you could get away with something simple like ds1618+ with a 10GbE card (or even ds918+). Things get more interesting when you need to watch something remotely. The DS918+ could transcode a couple of 4K streams on a DS video remotely, but nothing over the Plex. The DS3617xs CPU would freak out but it could transcode a  4K stream or 2 remotely over the Plex. Simple 1080p streams can be done on ds918+ easily even 8 or more. If you want to keep the network free you can stream 4K using HDMI cable. The HDR content will require HDMI2.0 which narrows the choice down to Qnap x82 series. The TVS-1282 i7 is also the best NAS for 4K transcoding you can get today. You can even install 10gbe card in it. run surveillance system and access it from mobiles.

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