And my final comment on documenting this issue.
It is completely solved in the current version of LineageOS, 20-20230803-NIGHTLY.
Issues discussed above were in respect of a much earlier unofficial version of LOS, and version 2.1.5 GMS-less Android from Fx Tec.
The current official LOS permits three codecs with these headphones, SBC, AAC and Qualcomm aptX, and they all work just fine, no sudden "suspend" disconnects.
So thank you very much LOS devs, and mystery solved!