@ChrisF i know you are quite experienced and have been on the forums for ages, so apologies if these are basics or repetition.
at 1.75, you're sort of in between, but height isnt everything, torso/leg ratio, arm length etc.
I'd always recommend sizing down. you can always make a small bike bigger, and a smaller bike with more exposed seatpost is more compliant, so your butt will thank you. Also, a smaller bike will likely come with shorter cranks and possibly a narrower bar.
a 54 is not a 54 is not a 54. scott, titan, merida seem bigger than treks and spez etc in the same size.
the bikes sites size guides are also generally garbage, and the salesmen no better as they just take what the distributor tells them. get on a few, read reviews etc.
a friend got a Speedster Gravel. he is your height. bike addict folks said Medium. a year later he is thanking me for convincing him to go for the Small.
the same crew tell me i should be on a 58cm. I am 188cm. I built a size Medium merida silex up with an 80mm stem, 44cm curvy Deda bar. fitted like a glove.
smaller bikes are also more fun offroad.
if the bike ends up feeling too big, you're paying for a new bar, new stem, inline seatpost, shorter cranks etc etc, and you'll be riding with a stack of spacers or a +40degree stem
