[Icecast] Transcode lossy to further reduced lossy to stream over Icecast

[D.T.]

Situation: 
 * remote virtual server with very little storage (estimate: I can
   spare about 40G for music)
 * local music collection of ~80G in all sorts of formats - lossy in
   varying quality, some lossless too

Vision:
 * stream my whole music collection randomized so I can listen to it
   anywhere

Plan/Idea:
 * Locally transcode everything to one format that results in files
   that are 
    - small enough to fit on my server, altogether
    - have a reduced bitrate for streaming
    - can be streamed as-is without further transcoding
 * Upload
 * Set icecast up to do just that (this I know how to do)

So I'm asking advice for the transcoding. What's likely to give the
best results with already lossy sources, and at small bitrates?

According to these documents:
https://trac.ffmpeg.org/wiki/Encode/HighQualityAudio
https://wiki.hydrogenaud.io/index.php?title=Transcoding#Lossy-to-lossy_transcoding
it basically comes down to Fraunhofer Institute's FDK AAC, but the
articles are dated.
Opus is supposed to be good but I always have the impression it doesn't
deal well with loud/grungy/fuzzy/guitarry music, esp. at low bitrates.

What do you say?

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