Quick and easy way to batch convert multiple HEIC to png or jpg on macOS?
This answer is what solved for me. It's based off @user3439894's extremely useful comments
To convert all .HEIC images in a directory to .png
Open terminal. cd to the directory where the .HEIC files you want to convert are stored.
for f in *.HEIC
do sips -s format png &quot;${f}&quot; --out &quot;${f%.*}.png&quot;
done

If you want to convert and resize images
The following will convert and resize images so the maximum height and width is 400 pixels (but you can change 400 below to any value)
for f in *.HEIC
do sips -s format png &quot;${f}&quot; -Z 400 --out &quot;${f%.*}.png&quot;
done

Reference

sips documentation: https://ss64.com/osx/sips.html