Suspect your problem is the cable housing itself. From the photos it looks like you`re using compressionless housing which is extremely stiff and refuses to take small radius bends.

It was actually quite common for Shimano SIS systems to use spiral wound cable housing to terminate runs to rear derailleurs for this kind of installation. The other area they were commonly combined with compressionless cables was the short length at the top of the seat tube if a small radius bend was necessary. That will likely solve your issue without the need for tie wraps.