For 1080p60, the foliage in BDO is going to kill your quality output, unless you are pushing towards 12k+ bitrate, which Twitch simply won't allow.
You will almost certainly notice much better quality output in other games that don't have grass/foliage like BDO does. That being said, 6k bitrate is still very low for 1080p60 regarding streaming games that are high motion/lots of effects/foliage, etc.
You could even try dropping to 720p30, the most important thing is to use the slowest encoding preset your PC can handle and have Lanczos Filtering in Video Settings, along with the highest bitrate your internet can handle and are allowed by your preferred streaming service provider (6k for Twitch, x amount for YouTube)
It may come down to how Twitch is trans-coding your stream for viewing as well.
If you have a lot of subscribers and a Twitch partnership you will get better quality output as it is more beneficial for Twitch to have a streamer with a lot of subscribers/viewers have great quality live streams than those that have less subscribers/viewers, as it means more people see advertisements. Unfortunately there is not much you can do about this, except to keep working at it and slowly build up a larger viewing audience. Ask your viewing audience for feedback as well, after all they are the ones watching your stream.
Note: You will find the same issue in regards to your stream being trans-coded with YouTube, though the allowance for higher bitrates really helps to negate that, a lot.