A6 Alloy options? (B8/B9 S4 donor cars?)

Any recommendations on 18" / 19" alloys?

I've got 18's on there right now - I gather that my stock setup is: 245/45R18 8Jx18 ET39 5x112 66.6CB. According to Audi, 18" is the smallest I can get to fit over the V6 TFSI discs and calipers. There is some appeal to keeping the 18's (my summer tyres, Pirelli P Zero's still have decent tread), but I'd consider 19" too.

I gather from the "wheel-size.com" site, that for 19's, I'd need: 255/40R19 8.5Jx19 ET45

From the same site - it looks like the B8 S4's are: 8.5Jx19 and ET43, so close enough, BUT DOES EVERYTHING ELSE MATCH? If so, I think they would make a good donor setup as there's a few S4/S5 owners putting 20's on their cars....

Same question again for the B9 S4: as that that might work better as their 18's are a closer match - they have a 8Jx18 ET40 - annoyingly, that would be 1mm narrower though, so I might look at some hubcentric spacers, if I end up with those.... but would everything else fit a C7 gen A6?

My main question is: which cars would have the correct CB and decent offset (same or very slightly wider) to a C7 gen A6?
Just to be clear - even if I "keep the 18's" - I intend to get a 2nd pair, to make it easier to swap tyres each season.
I think the main thing I need to confirm: that the A6 C7 needs a CB of 66.6mm? (not the older VAG/Audi style of 57mm)
I copped out and picked up a replacement set of identical alloys (18" 5x twin spokes) - my summer tyres still all have nearly 6mm tread (as do the winters), so it would have been a waste...