00:33:54.788: Output 'adv_stream': Number of dropped frames due to insufficient bandwidth/connection stalls: 1001 (18.6%)
A speed test is only slightly meaningful, as those tests tend to drop low scores (which is contrary to what you need to know of minimum throughput requirements applicable to livestreaming). Numerous previous discussions of this
And having a certain upload ISP connection is NOT the issue. Your could have a fiber 1GbE ISP connection and still have a problem. In this context, having a certain amount of bandwidth available to you for streaming is the real issue (not the ISP link itself, if all of that bandwidth isn't available to you the ENTIRE time of the stream). And there are plenty of potential other sources of upload traffic on your LAN (depending)... or there may be other things running on your PC in the background interfering (Killer networks driver/software. certain security software/settings, file sync software, malware, etc... LOTS of possibilities, none having anything to do with OBS Studio... it is just that OBS Studio is likely the ONLY upload jitter and latency sensitive traffic generator you deal with, hence your noticing the issues with OBS Studio, but that is correlative, not causal.
However, all those browser sessions may be an issue on top of whatever else is going on? maybe?
1. go thru FAQ on this site (see my .sig) regarding network/connection issues
2. Check ALL background processes on your PC to make sure no unexpected traffic. Check traffic on your router to see if it matches your expectations (is appropriate)? ie, if you aren't streaming, but there is already plenty of upload traffic, then you have some research to do
3. Is your ISP a stable wired connection type, or cellular based (which can have greatly varying bandwidth moment to moment)?
4. Create a new scene collection. No browser sources/scenes, only you webcam... nothing else... no chroma filtering, audio filters/effects, etc. super simple... now try a test. *If* bandwidth still an issue, and LAN/WAN link is ok (per real-time monitoring) then you probably need to fix your operating system