I've had the 6-series for about 6 weeks now, and it's fantastic. Also gets plenty bright, so you won't have any issues there.
The picture settings are a little funky, but if you're going to use the built-in Roku app then it shouldn't be as tough, since you're only using one input (picture settings don't carry over to each different input, so you'd have to set it up for each one). However, you will have to set picture settings for SD, HDR, and Dolby Vision, and you can only change each when you're viewing that content. A little annoying at first, but the picture is fantastic, and it's nice to alter the settings based on what kind of content you're watching.
I also upgraded from 50" to 55" and it makes a huge difference. I'm glad they only make the 55" in the 6-series or else I would've settled for 50".