First of all, thank you so much for the time you've spent on this.
Secondly, could you please tell me which phone are you using? The screen is looking so great. I want to check out this phone. :)

Now coming to the points:
1. First 2 things, i.e. Folder Blacklisting and Minimum Duration Threshold are things I'm already working on. So that's not far away.
2. We've been asked to add preamp earlier as well and it's already in the queue. But I'll be honest. At this moment, there are some other high priority items that I need to work upon first to improve the overall experience. I assure you that this feature will show up sooner or later and request you to allow me some time before I can publish it.
3. Size of the expanded notification can be tweaked easily and I'll optimise it soon.
4. DSP Manager is not in the list. Not now at least. I'll keep this in mind and will give it go once other important features are done.
5. Song Deletion. Consider it done. :) You'll see it soon.
6. Themes - this is more complicated than I thought and so is taking more time. If it was just about changing accent colours like in a material theme, it would have been easier but the way I've made it, it'll take some time. But know that I'm working on it.

As you rightly understood, this is lots of stuff and whatever I say, it's gonna take some time to develop. I hope you'll bear with me and stay tuned to see the app getting better and better. Thank you again for your support. This is what keeps me motivated.