I found the resolution. When I was configuring this Navidrome server I removed the default transcodes and only left my modified mp3 transcode. My thinking was that sticking with MP3 would improve compatibility, so I removed the others.
This is why when Symfonium requests the transcode to Opus, Navidrome provides the raw format. It was no longer capable of transcoding to Opus and I was unaware that Symfonium is designed to request Opus.
Adding back a transcode command for Opus on the server side solved this issue:
ffmpeg -i %s -ss %t -map 0:a:0 -b:a %bk -v 0 -threads 0 -f opus -
Thank you very much for helping me work through this issue. I’m not sure how long it would have taken me to discover that it was requesting Opus had you not mentioned it. I’ll happily buy the app several times over for myself and my friends.