I ran the numbers of the B'Twin against my Bianchi Impulso Allroad. Size 50 is very close to the B'twin except slightly shorter top tube and slightly longer chainstays. May be able to keep the stock 90mm stem instead of swapping to 50mm.
Compare: B'Twin Triban 100 2017: S -VS- Bianchi Impulso Allroad 2020: 55 -VS- Bianchi Impulso Allroad 2020: 53 -VS- Bianchi Impulso Allroad 2020: 50 -VS- (geometrygeeks.bike)