HEIC vs JPEG

If you want your photos to be always JPEGs, change the Settings on the iPhone for the Camera to take JPEGs instead of HEIC files. See:

Using HEIF or HEVC media on Apple devices - Apple Support

1. Go to Settings > Camera.
2. Tap Formats.
3. Tap Most Compatible

You can also change the Settings > Photos > Download to Mac or PC. when you download photos from the iPhone the HEIC photos will be converted automatically to Jpegs. But this will interfere with the duplicate detection and you will have to hand-pick new photos to avoid importing duplicates.

If you mail a HEIC image file, the attached file will be sent as JPEG. When mailing, you can select one of the sizes small, medium, large, original.

Bear in mind jpg files will take up twice as much space as HEIC.

As an alternative to storing JPG on your mac, (or making your phone take jpg images, which will halve the numebr of images it can store), you can just export the images you want to send from your photos app as jpg to a folder first, and sent them from the folder. Then delete them from the folder after they've been sent.