Your logfile does show your CPU is struggling a bit, as well as your GPU:
19:45:13.317: Output 'adv_stream': Total encoded frames: 189754
19:45:13.317: Output 'adv_stream': Total drawn frames: 189754
19:45:13.317: Output 'adv_stream': Number of skipped frames due to encoding lag: 15062 (7.9%) <--- CPU
19:45:13.317: Output 'adv_stream': Number of lagged frames due to rendering lag/stalls: 15 (0.0%) <--- GPU
The above was from your 720p@30fps test, at 720p@60fps it only made it worse.
I think you might have to re-do you scenes a little, you have a lot of sources tied to one scene (Scene '0' in particular) Some of those sources may be conflicting for your stream performance.
Good thing to note is you should be able to increase your bitrate as there is no issue in regards to your connection to Twitch and your upload speed.
If you haven't already I think you should take a look at the guide
@NerdOrDie created, it contains valuable information:
https://obsproject.com/forum/resources/full-video-guide-for-obs-studio-and-twitch.377/