Opus is a free and open source lossy audio coding format developed by the Xiph.Org Foundation and standardized by the Internet Engineering Task Force, designed for efficient low-latency encoding of both speech and general audio. Due to its lower latency relative to other standard codecs, Opus finds specific use cases in real-time interactive communication for low-end embedded processors. Opus replaces both Vorbis and Speex for new applications.
In listening tests around 96 kbit/s, Opus shows slightly superior quality compared to AAC and significantly better quality compared to Vorbis and MP3.
Sounds like you have a nice project ahead of you to rerip all your CDs to FLAC, and then convert to Opus for your devices.
I did that a few years ago, I recommend dB Poweramp to do the ripping and converting.