I’ve used win 10 and a 1060 to play the files using a madvr setup I found on overclockers. It seemed to work well.
I’ll have a look at these Veto boxes. Will it just play the files correctly? Rather than need lots of tweaking? Can it do Dolby vision?
Yes it will play HDR10 files without tweaking, but not DV as for Dolby Vision that is a complicated matter, there are multiple versions of DV & licensing to factor in, currently either the player built into a DV capable TV or an AppleTV 4K running MrMC is the best way to play DV content (even then there are limitations with dual layer DV content).
The newest Nvidia Shield Pro has Dolby Vision support so MrMC or Kodi on that may be able to play DV content in time on that.
Technically no but effectively yes, HDMI 1.4 does not have the bandwith for 4K HDR, you could push HDR out at 1080p but few devices support that.
Also I have one of these:
NEXBOX A3 Amlogic S912 Octa-Core 2GB RAM 16GB ROM Android 6.0 2.4G + 5.0G WiFi Gigabit LAN 4K HDR TV Box
Bought it a couple of years ago. Any good?
Thats a tablet Android box not actual AndroidTV like the Shield, the S912 is a capable chipset however the limiting factor is the software.
You could try loading
CoreElec a modified version of Libreelec for set top boxes, you can load it onto an SD card I believe, there is a generic S912 option and a Nexbox A1 available, the Nexbox build might be worth a try as it also uses the S912.
Whether that works with HDR is unknown to me.