"but rather only" is a correct and usable phrase in written English.
You can use it to show emphasis, or to contrast two ideas. For example, "I wasn't scared of the roller coaster, but rather only excited."
"This tells me that China has no friends but rather only interests".
The Noel Canning company argued that the Constitution did not recognize intra-session recesses but, rather, only inter-session recesses.
Not all males of enlistment age will now be required to serve, but rather only a small proportion.
It is not responsive to what we want, but rather only to the special interests that now dominate it.
Most of the women in the wards were not tested even after it was clear their husbands were H.I.V.-positive, but rather only when they became seriously ill.
"We are not disputing King's college data, but rather only asking only that in future more rigour will be applied to public statements," Johnson wrote.
In this example, the opponents are not protesting against the project itself but rather only the concrete circumstances.
"Tape mode" isn't available across the whole of YouTube, but rather only on select videos.
Sinclair and other industry experts agree that this law cannot be enforced completely, but rather only to a certain degree.
RumEnKF differs from EnKF in that it does not store the entire ensemble, but rather only saves the first two moments of the ensemble distribution.
So, a score of 550 on the TOEIC, she said, was not the major factor in hiring, but rather, only one of many factors.
