The rim in question. External 25 internal 17.9. I guess this is the narrowest out of the modern rims..?
Tire in question is Conti GP5000. Luckily these come in 23 and 25mm.
I think best is to put some spare wheels and brake calipers on and do the measurements...was hoping someone could confirm so I could maybe go ahead and order the wheels whilst away for 2 weeks