Is it possible to change rider / bike set up after initial setup ?
Press ‘T’ I think it is while the app is open.
Also it can be done at the end of the race
Yes, just press t to open up rider customization. For more shortcuts and info check out the unofficial guide: https://docs.google.com/document/d/1WZ3rvtip6WNjGOszANnpZUYEAnkAlVvRqa6MvnHhst4/edit