Thank you for your reply, Carbon_Dude
You're correct - those 88cm (34.65") for my own inseam seemed like a bit much for my total hight, so I did a remeasurment. And thank God for that. This time I measured only around 80-81 cm which seems more right for my 174 cm (5'-8.5") height.
Based on this new measurment, I guess that a medium frame (17,5") makes the most sense - right?
The tougher one is my father though, since he is 56 years old and not as flexible anymore.
Right now both him and I ride old 26"-wheel bikes with ETTs of 60cm (23.622") and a 100mm stems and he feels like reaching a little for the handlebar. A large (19") CS-057 has an ETT of 62cm (24.41") som worried that this would be to large for him.
Maybe combined with a shorter stem it would be ok?