people I know with actual military experience,
If this was US-style "shock and awe" warfare - with massive air and artillery support - the casualty-to-POW ratio would seem believable... but that's not what the Ukrainians were doing, were they? To get that kind of ratio through mainly infantry warfare (mechanised or not) seems extremely dubious.
Mass surrenders are fairly uncommon events in modern warfare
I wouldn't say that - just look at the mass-surrenders during the US invasion of Iraq.
My guess is that - war crimes notwithstanding - is that Russian casualties have been grossly inflated for propaganda purposes.