What is the best way to guarantee that the person I send an image to from my iPhone 10 receives it in full quality?
Pending on sensitivity of photos and privacy preferences one can use iCloud link instead of attaching file to e-mail (or sending using messaging app). iCloud Mail will use iCloud link for files over 20 MB anyway.
iCloud links expire automatically after 30 days (you can stop sharing manually whenever you like) and anyone with the link can view them. More information in Apple support document: Share photos and videos on iCloud.com