Non-partnered casters are advised to use 2000kbps as a maximum, to minimize viewer buffering per Twitch's user metrics. Above that, more and more people will increasingly buffer. A 720@30 stream on x264 Veryfast will provide watchable video (some artifacting, but not unlivable levels) at 2000.
3000-3500 will tend to put people into buffering hell. Even 2500 is borderline, and not advised.
Do you have a link to the VOD showing the mic issue, and a couple of timestamps to show the variance?
You're running slightly lean on the audio side, commonly 96 or 128kbps AAC is used. Also showing some mic timestamp skew in the logs, but not bad. Possible the low bitrate could be interacting with the late mic audio to cause something, but that's spitballing. Are you running your headset in surround mode? OBS can only handle stereo/2-channel audio, and if the mixdown from surround is having issues, that could be interfering as well.
As far as restreaming goes, to point out an alternative, it's pretty simple to set up an nginx-rtmp repeater on something like a DigitalOcean linux droplet. Keeps the entire stream path under your personal control at that point.
