Very sorry for the late reply,
High Priority mode is an option in OBS Setting ---> Advanced---> Process Priority.
From your logfile:
11:37:45.823: Output 'simple_stream': Total frames output: 33737
11:37:45.823: Output 'simple_stream': Total drawn frames: 33779 (33831 attempted)
11:37:45.823: Output 'simple_stream': Number of lagged frames due to rendering lag/stalls: 52 (0.2%)
11:37:45.824: Video stopped, number of skipped frames due to encoding lag: 60/33776 (0.2%)
Your dropped frames are very marginal, which is good news.
If you have the preview pane of OBS turned on, try turning that off as it uses a small amount of resources which might be pushing it over the edge.
If you view your stream live, try minimizing the browser.
Can you try disconnecting your secondary monitor.
Try using superfast preset, or even ultrafast preset. The quality output will degrade somewhat, it will however reduce the strain on your CPU a bit.
Note:
I really think you should be able to run the game at 60fps whilst streaming at 720p60fps with in-game settings high or above, given the hardware you are using.
Not entirely sure as to what is going on. Perhaps look elsewhere for the cause of the issue? Take a look at your CPU and GPU temps plus load whilst gaming and streaming. Maybe a case of thermal throttling slightly? I personally use HWiNFO64 to monitor CPU temps, etc along with MSI Afterburner for GPU temps, etc and have On Screen Display via Riva Tuner Statistics Server (RTSS, coupled with MSI AB installation)