I hope you take this the right way, but I'd do a bit more research on your plans.
I've just built a Plex Server to replace my 918+ and CPU choice and OS are important if you're doing lots of transcoding.
Depending on bitrate, and what you're transcoding down to you *might* be able to do 2 4k streams using a modern Intel CPU with Quick Sync. However, if you can't, you need an Nvidia card and Ubuntu is the best OS to offer up the Nvidia card. Using something like Freenas you'll have struggles unless you're super prepared for hassle getting the functions from the card to pass through to the Plex container.
I've gone for a 11600K Rocket lake as its got one of the better intel GPUs for transcoding, and where that fails a nice high single thread performance with 6 core/12 thread to fall back on....
Further heads up, you can't use LTS Ubuntu 20.04 with a Rocket Lake CPU as the kernel doesn't support the new intel iGPU in Rocket Lake - you need kernel version 5.11 or higher. Using 21.04 works just great.
A 2070 should be able to do 2 4k streams OK though - but again this depends on the source file and what you're expecting it to go down to. Read up on how to configure plex properly for transcoding (EG using RAMDisk for transcoding temporary folder, etc).
It took me a week or two of reading threads and working out what I was going to do to finalise it all, however now its sorted its good.
I'd recommend you also make sure you have the best possible solution you can at home for the tvs you're streaming to. I've got Shield TV Pros (not tubes, they dont like 4k Remux very much) and that is allowing me direct play and thus keeping my at home streams to simply network traffic loading.
