You have already answered the quicksync question about your 4790 with the chart you posted, you need at least a 7000 series intel cpu with 600 series igpu in order to support 4k hevc decode/encode.
The equivalent chart for nvidia @ Video Encode and Decode GPU Support Matrix | NVIDIA Developer.
a gt 710 is not going to help 4k transcoding, a bare minimum would be a GM206 model, GeForce GTX 750 / GTX 950 - 960 Maxwell (2nd Gen) GM206 2nd Gen
a second bare minimum would a quadro p400, which can handle ONE 4k transcode.
the p400 2gig vram (half height) and p2000 5gig vram are the only single slot 4k transcode usable cards that I am aware of.
else, you are looking at a gtx 1050+ with as much video ram as you can find, each 4k transcode uses about ~1.4gig of video ram.
So in your situation (and most everyone’s to be straight), an intel 7000+ with quicksync will likely be the cheapest, simplest, most energy efficient, server for 4k transcoding.
as to the question of waiting for rocket lake, since they aren’t yet available, that is anyone’s guess, but if you have time to wait, then I personally would wait, simply to avoid buying 10th gen or earlier that already going to be obsolete.
if you need something right now, get a 10th gen of suitable power/affordability, and you should be set for a long time.
While AV1 etc is enticing to wait for, no doubt the software support will take even longer, and by that time, you could add a future gpu that supports it, if it is that important to you.
Besides, its not like 4k blurays are suddenly going to change from hevc to av1.
