I do exercises to strengthen my core daily but due to some pathologies I don't have well developed muscles and I will never have them (I can do thousands of exercises but my core will remain weak).
I have little flexibility in my back and have been told an endurance bike can help.
I had two bikes with endurance geometries, an emonda with h2 geometries and a cannondale synapse and I found a good compromise, then I tried an ostro vam size 54 (55 horizontal tube) but I can't find the position, the same problems persisted presented when I took a specialized tarmac. Beyond the geometries, my feeling has always been that of having a bike that is too big.
For example, all biomechanics say that my ideal size would be a bike with a 54.5cm top tube and an 11cm stem, but I've never always had the feeling of being better on a smaller bike ... size 52 with a 53.5 top too if the height of the head tube is lower precisely because the longer I stretch the more I have back problems