A2DP itself does support bidi, faststream is a A2DP BIDI profile (it’s SBC, it’s bad because of really bad bitrate constraints for some reason though) AptX-HD actually support aptx-ll but I find most devices just bug out when you try it. opus also supports bidi on pw.
bandwidth limtations are not an issue with bluetooth itself, you can reliably maintain 300-600kbps, which is more then enough for good quality bidirectional audio. even when using asynchronous modes, you still usually have enough bitrate for audio that would be considered fine.
im not sure why it’s not more popular in modern devices, IIRC all the patents on the subject have ended, could be a limitation of low end radios