Could be LOTS of things, including those NOT captured in the log.
For example, are you monitoring ALL other devices using Internet bandwidth while you are streaming? are you monitoring your Internet connection throughput (separate from OBS stream)? Have you optimized your Windows setup to eliminate unnecessary background tasks and services which might interfere? Are you using Task Manager/Resource Monitor to oversee resource utilization on your system to make sure you aren't bottlenecking?
With that said, you have an older CPU and are re-scaling, aren't running Win10 v2004,
Others may mention running OBS Admin as admin (which is generally a bad idea, unless resource constrained, which you likely are..)
and you are running at 60 fps? try dropping to 30fps to get a stable stream, then see about increasing settings
and to minimize resource load on PC, have Base & Output Resolution be the same
This in the log indicates you have a network issue
21:36:15.728: Output 'simple_stream': Number of dropped frames due to insufficient bandwidth/connection stalls: 1119 (3.3%)
21:39:33.907: Output 'simple_stream': Number of dropped frames due to insufficient bandwidth/connection stalls: 1109 (9.9%)
Now, is your PC completely overloaded, and that why network issue, or is network its own issue? can't tell from this log
I'm not a gamer, but that is a quick observation. Some of your other settings may not be optimized for gaming on an older CPU. I don't following gaming recommendations, so can't comment. But if you search this forum, you'll see lots of comments about Aero Mode, Game mode, etc. And if you are using OBS to stream (and can record), why also using
21:21:02.678: Game DVR Background Recording: On