Why put the umbrella so subject is 'lit from the back'?!
In dim lighting conditions there ought to be INSUFFICIENT light to photography the front of the Bride's (presumably) expensive dress to show off any details of its adornment, and especially no ability to discern any detail at all of a dark suit the Groom might be wearing...it looks like a black detailess mass in the photo.
Yes, 'lit from the back' would help to delineate and separate the B&G from a dark detailless background area behind them, but it would be better to capture a little of the background ambient lighting, albeit somewhat dimmer in brightness from the main subjects...i.e., the process known as 'dragging the shutter' that is commonly used by professional photographers.
As for 'advice of settings' you simply need to set shutter speed and aperture and ISO so that the ambient light contributes SOME light for the background,
while eTTL flash automation properly illuminates your main subjects.