Any question answered...

would embankment be better or worse in terms of congestion?

Embankment evry teim.

Maybe not particularly chilled, but leaps and bounds faster than wading through the alternatives.

Embankment is sometimes nightmare-ish. But it's very hit and miss.
Having said that, river views trumps views of Chelsea tractors every time.

What a lot of cyclists don't realise is that you can legally ride on the footpath by the river along the most congested stretch, and go under the bridges, thus passing all the stationary traffic.