Flash does not have to be invisible. Complement the overall light. Getting the speedlight off the camera. a good start. Agree that the flash should not call attention to itself, hard to deny. Sun is the ultimate single source. Big, diffuse and directional. When you don't get sun you improvise. Direct flash used to be more common and less distasteful. Because of the shadows on the wall thing first of all. Shadows. bad. Eyelids closed, way bad. Use flash like garlic seasoning, sparingly. A shadowed courtyard for a group, not bad. Flatness of light, so so, so help it out... Add some flash, maybe a little better depends on taste. Interesting. Never has flash been easier or so apparently disliked. Funny. Modern cameras w communication capability do a super job of balancing flash and ambient and think for you. A flash bracket used to be part of the average kit, getting direction to side. Not so much anymore. . Little pop over covers to a speedligtht to diffuse and spread?. Ooh. I do not think so, tried them, minimal help.not sun sub.. (Peter Gowland did some great show- and- tells for Vivitar Old but useful. Look him up) If a wall reflector fills the bill use the wall. It is cheap and available. Subject is as broad as photo itself. A full semester thing.