It is not because BRS deals with all Rasas, that any of the 4 Rasas (Santa, Dasya, Sakhya and Vatsalya) will be cultivated by GV followers of Sri Caitanya. The summary description of these Rasas is there simply because they are part of the dynamics of Madhurya Rasa which is discussed in detail in UN, the advanced study of BRS. There is no advanced study of the other Rasas.
Also, it is not because UN deals with Sakhi Bhava, that this will be the goal that GV should aim for, but simply because Manjaris should know all the details of Sakhi Bhava to nurture their own Bhajana. As servants of the Sakhis, the Manjaris must be deeply familiar with the dynamics of Sakhi Bhava.
Also, to affirm that a Sadhaka follows Manjari Bhava because the union with Krsna is something "impure" is an absurd and primary. Any Sadhaka knows that the delivery the Sakhis make of themselves, of their bodies is completely pure.
A GV Sadhaka follows Manjari Bhava because this comes in disciplic succession from ..... Sri Rupa Manjari !!!
Also absurd is to say that Manjari Bhava would only be an initial phase that would then pass to Sakhi Bhava. No, those who long for and reach Manjari Bhava are eternally Manjaris.
Regarding the gradation of the Gopis Prema given by VCT in the UN commentary. Yes, Priya Sakhis have more Prema and can be cultivated (followers of Nimbarka and Vallabha do that). But then we have VCT himself and all the GV Acaryas to emphasize Nitya Sakhi. A very strong influence even for those who might have a tendency to other Rasas !!!
Then to a GV, because his relation with the Guru is eternal, Manjari Bhava is the highest.
infirma mundi elegit Deus