Welp, I can report that I have managed to disassemble my venerable OnePlus 6T phone, replace the battery & find I have the wrong USB socket replacement (prob for my now obsolete 3T instead), but managed to clean the USB connector contacts, removing the need for a replacement, & reassembled it successfully. Subsequently, I (for the umpteenth time) managed to do a painless major version upgrade of LineageOS to version 22.1 (from 21.1). Amazingly, everything is functioning! Big sigh of relief.