There’s heaps of good suggestions here, but I wanted to recommend something a little different - you seem to be looking for knowledge as much as suggestions. Here are a couple of bbc podcasts that introduce you to snippets of music - you can get an overview of what’s out there and start to figure out what you want to follow up. There is almost always a link to imusic or Spotify on the website.
“
The Listening service” has all kinds of odd themes (with a pretty wide variety of (usually classical) music in each episode and is the main thing I wanted to post. I can’t think of a better way to learn about classical music than this.
Private passions is a biography podcast with (mostly English) writers/actors/activists etc discussing music that they’re passionate about.
For a detailed look at individual composers, “
composer of the week” covers a single composers life over 4 programs
This one might not be metal enough, but you were talking about impressionist music so I’m adding it anyhow. “
Night tracks” always has an interesting selection of classical, electronic, and world music, which I find a good way to expand my very fixed musical tastes.
I’m also throwing in
Australia’s favourite instrument - less for the actual programs, which are mostly no longer available (but were brilliant and worth searching for) but for the list of 200 instruments of all kinds to inspire anyone whose special interest is music.
https://www.abc.net.au/classic/classic100
Anyone else have favourite radio/podcasts like these?