I did find the service very useful for the 3ish years I used it, but I didn't jump to premium, and kinda stopped using it.
Curiously, what makes this service different and how will you ensure "100% perpetually free with no strings attached"?
And to /u/bartproost - good luck with your new adventure! Your note on monetization reminds of myself 4 years ago when I'd just started with MusicButler. Luckily I didn't make any promises about it being free forever because, well, hobby projects have associated costs too.
Here's a demo of how I use Brew.fm to keep my 2 most used playlists up to date: https://www.youtube.com/watch?v=Nh2Ognb4PgU.
I have no plans to monetize the tool. I'm scratching my own itch here and am very much at the mercy of Spotify. I'm hoping to keep this one a hobby project.