I’ve sorted this now. Was a doddle. Convert to mp3 using OpenAudible; import into iTunes; and then sync to Fenix under Mymusic. Had to tell iTunes to change them to Audiobooks mind; it thought they were songs. Easy when you know (or work out) how!
I connected Garmin to USB to update firmware for whole night. In the morning disconnected it and get message "Content Expired - Update your downloads to continue offline use" Connected it again, disconnected and updated firmware to the newest version. Garmin Fenix: Fenix 7: All-purpose Suitable for many activities Compatible with all new Garmin products Attractive style options: Active runners and cyclists who also ski, golf, and more: From Garmin From Amazon: Garmin Forerunner: Forerunner 945 / 955: Basic and functional $200 or more less than the Fenix Simple to use for triathletes 5ITJ.