I tried to also use the iOS shortcut.
I used the last one available :
https://www.icloud.com/shortcuts/f463da8a7ab14e71999f2d4d9e14463a
I replaced the URL from your exemple with My script URL.
On the iPhone, I can choose the message template, and verify that informations are correct..
But when I send it I get this error message : " De non valide. La valeur fournie pour De dans Envoyer un e-mail n'était pas valide".
Seems the "from" value is the problem.
I really am a noob in shortcuts so i can't help debbuging right now.
[EDIT] When I use the shortcut from my Mac, i do not have the error message, but nothing is received and the log seems empty.