Perhaps I'm brute forcing something that has an easier solution, but I don't like my uploaded (airdropped) iPhone photos to be in HEIC format on my Mac. So I have a Downloads folder rule in Hazel that runs the following three scripts for every uploaded file:
/opt/homebrew/bin/convert $1 $1.png
VAR=$1
/opt/homebrew/bin/rename s/HEIC\.// $VAR.png
/bin/rm $1
[I guess could all be one script - can't remember why I split it into three separate scripts.]
Now to make all that work, you have to previously have installed 'brew' and then install 'imagemagik'. But this works like a charm and is very reliable.