For anyone reading up on the error “Unreal Engine is exiting due to D3D device being lost” and finding this thread - reducing the GPU Clock solved this problem for me.
I first spent several hours testing everything from switching fullscreen/windowed, lowering graphics settings and capping the framerate, reinstalling my GPU drivers, tweaking the TDR Delay value, monitoring the GPU temp (below 75C) and memory, etc. My GPU was never overclocked, many other games working fine. Still got this fatal error from Unreal Engine after several minutes into the game.
Then I finally lowered the GPU Boost Clock by 170Mhz and the error never showed up again even on Ultra settings with 144Hz framerate.
I used Asus GPU Tweak tool, sure MSI Afterburner would work as well.
UPD. With MSI Afterburner I had to set Core Clock to -500 to make my RTX3070 stay below 1600 Mhz.
While my GPU was below 1600 Mhz the error never showed up and Unreal Engine worked stable all night long in Conan Exiles.
In principle, for different graphics cards try adjusting the Max Clock (Mhz) value so that your Max GPU Clock goes down by about 10%.