"rather successfully" is a grammatically correct and commonly used phrase in written English.
It can be used to describe how someone or something has achieved success, but admits that it may not have been a perfect or complete success. Example: "Despite facing some challenges, the team rather successfully completed their project on time." Another example: "Although she was nervous, Sarah rather successfully delivered her speech in front of a large audience."
Detailed theoretical calculations have rather successfully predicted the spectra of the best-observed nebulae.
It has redefined itself, rather successfully, as a broad entertainment network.
Green has made that kind of tough decision about the quarterback, rather successfully, a number of times.
Formerly just a base for heli-ski operations, Revelstoke is now marketing itself – rather successfully – as a new resort.
Nor has he abandoned casual games, as he appears regularly (and rather successfully) on the Playchess server, under a pseudonym, to do battle with the leading lights.
The film develops the idea from the book that Rachel is an aspirant artist; she's often seen sketching, rather successfully, even when half-cut.
But the deregulated, privatised, market-oriented world she helped to create - rather more successfully than his own efforts - had triumphed almost everywhere.
For many years, Britain imposed strict controls that helped keep its costs down rather more successfully than those in other rich countries.
Another Lyttelton production which used dancing as a comment on warfare, rather more successfully, was Katie Mitchell's Women of Troy, in which the chorus of captured women danced with each other in the grim surroundings of the industrial warehouse where they'd been herded to await allocation.
Aircraft were flown off and, rather less successfully, landed on this deck.
I'm a little afraid for Tommen, but Cersei is more Medea than Lady MacBeth, avenging betrayals with the worst possible acts, rather than successfully plotting her way to power, which makes her a less-than-ideal candidate.
