IanH - I am indeed quite short (172cm / 5'8") with not particularly long legs or arms, so what you've said makes some sense.
I'm hesitant to offer yet another view, but
I am 1 cm shorter than you, and with proportionally short legs, and long upper body.
this is all 5 foot 7 and a half of me and my stumpy legs next to a 17in MTB
I have a couple of Mercians, both of them 54cm seat tube, I also have and have had other bikes ranging from 52cm up to 57cm and they all 'fit' although the relative proportion of exposed seatpost and stem, and stem lengths (and bar reach) are different, the contact points are broadly similar between all the bikes, with only minor variations commensurate with their intended use.
Some photos below, purely as a reference point from someone of a similar height and how setups on different sized frames look. I'll make no further comments as I think you need to do some fiddling and testing to work out what's going to work
for you position wise.
Singular Osprey - 52cm - my main Audax bike, fits perfectly but aesthetically I think it looks a wee bit small, stem is at max height and looks lower than yours.
Genesis Equilibrium - '52cm' as described by them, but 49cm ST and 53CM TT.
setup as winter trainer/club run bike
*The headtube on this thing is far too long, I cant go any lower with the stem without going for -17deg
Bontrager Road Lite - setup for fastish road rides, listed as 'small' but very similar to the Genesis, 49cm x 53cm but with slightly shorter headtube!
Spa Cycles Ti Audax - 52cm - setup for audax, probably could have bought the 54cm but was worried about headtube being too long.
Condor Pendio - 53cm - setup for fastish road rides when I want to ride old steel in the wet
Again wiht this one I think it's on the verge of looking too small.
Woodrup 54cm - setup for fastish road when I want to ride old steel in the dry
Mercian Classic - 54cm - setup as SS Audax
Mercian KOM - 54(.5) cm - setup as relaxed Audax, note the extended headtube...
Carlton 57cm - setup as commuter