I'm a bit late coming to this discussion, but I like the idea of rediscovering old threads.
How do we take the term Avante Garde? Do we mean experimental? If so a lot of Beatles music was that, but usually it is interpreted as not following standard structures. In music it might be that it doesn't follow harmonic and melodic patterns. So Two Virgins was Avante Garde in that sense. Yoko did a lot of happenings events, so they were art theatre almost, rather than having aesthetic qualities.
John's music after meeting Yoko was I think less musically inventive than his music with the Beatles, even the jarring Plastic Ono Band seems musically more simplistic and even lyrically he became more straight forward and less poetic, the subject matter was more obvious and a lot of the time it was directly about John and his relationship with Yoko. Intense but not necessarily astute.