The Bluetooth classic, the variant which is typically used for audio and handsfree use cases, does not support address randomization. However, Bluetooth classic device does not necessarily need to be in discoverable mode for paired device to be able to connect. Typically they are discoverable only when they are set to some kind of pairing mode.
Bluetooth LE, which is used for wearables like smart watches etc, supports address randomization and it has mechanism for finding paired devices even though the address changes. Bluetooth LE devices must advertise for other devices to be able to connect. Many headsets nowadays use classic Bluetooth for audio streaming and handsfree but in addition they use Bluetooth LE for configuring the headset through their app. That is why the headsets are often advertising all the time.
As a side note, contact tracing apps are making our phones also to advertise. Fortunately they are using address randomization properly to ensure phones can't be tracked.
Most likely no. LE Audio requires controller support for Isochronous channels which is major part of Bluetooth 5.2. Usually firmware updates for bt chipsets are more like bug fixes than new features.
Comparing Zephyr and TNEO is like comparing apples and oranges. Zephyr is full fledged operating system with networking etc while TNEO is more like basic building blocks of os for those who want to implement everything themselves.
I can warmly recommend Zephyr for BLE usage. It is up to date with recent bt specs and implementation is really solid.
As someone else pointed out it is not actually bandwidth problem. Reason behind this is that high quality audio streaming profile A2DP is one-way only and once mic is needed, profile is changed to bidirectional but low quality HSP. My workaround for conference calls is to use my MacBook's internal microphone instead of one of my Bluetooth headset.