I've never bothered with the "For You" playlists on Spotify. Always seemed too restrictive.

I just make my own playlists, add a bunch of tracks (at least ~75, but more is ideal), then just start adding the recommended stuff (or using the "Enhance" feature which does that automatically, but apparently that's not an option anymore for some people, though I can still use it for some odd reason).

Also, artists' radio playlists can lead to fun new discoveries.

Although, if I'm being really honest, I've kind of reached a point where I know what I like and don't really feel the need to expand in terms of new genres. It's just about finding new artists, which seem to actually pop up fairly often in my playlists, so I don't have much trouble with that.

Edit: Lol, I feel like this is actually unhelpful because you're talking about expanding your horizons, which I assume means discovering different sounds and genres, whereas I'm just like, "give me more powerful heavy metal lady vocalists, please".