Hey man, glad you like things so far! Can help you out a bit for sure:
1. To get a nice background blur, you'll want to use a blur texture as a background image, then ensure its semi-transparent with opacity:
Code example here: https://codepen.io/ariona/pen/geFIK
Typically for sidebars like the Groove app, I keep them at a fixed width, because if you think about it, what use does a responsive sidebar really have when something always looks good at one particular size?
Usually I shrink my main content, but keep something small like a sidebar static, then re-arrange things on mobile once I'm below 768 pixels.
Let me know if that helps!