Many non-Karas influenced chanters completely disagree with your statement, including the man who despises Karas the most in G. K. Michalakis. Specifically he had to say this when I asked him many years ago about isokratima practice in the Patriarchate:
What did Stylianos Tsolakidis (expressing Patriarchal practice at the time of Iakovos Nafpliotis) have to say about ison?
G. K. Michalakis: : [My teacher] Tsolakidis said "ena einai to ison". In slow pieces, whether the melody ascends or descends, the ison remains the same, fixed on the basis and does not move. The canonarchs [of the Patriarchate] were allowed to change ison on some occasions, in general by triphonia (fourth) or tetraphonia (fifth), ver rarely diphonia (third). Those who were not initiated would keep the same ison, which suggests that there might have been a form of double ison in the Patriarcheion. In fast pieces (canon), the ison would involve the known "M" for "mazi" or "melody", meaning that in the descending, below the ison part, the isocrate would sing along. In slow pieces, things are bit more complicated. Recall, there were Isokratis, Domestichos, Canonarchos, etc... The first canonarchos was the only one allowed to do what the the "boithos" =helper Domestichos would do: sing along the various conclusion cadences. The first Domestichos would sing along almost 100%, except for difficult passages. My conclusions are the following: The patriarchal choirs were never 100% complete, for at Iakovos' time, there was lack of personnel. But imagine we have 10 isokratis, 3 canonarchos of which the first (for one octave higher voices), 2 helper domestichos and the first.. what can we do? We may do double ison, but the second, non basis ison will be much more disecreet than what is done by EBX... the "entrance" and "exit" is done with "progressive changes of intensity", and the overall volume of a second ison is only about 30 to 60% of that of the basis ... it is discreet.
I add here what I had said elsewhere: when the number of chanters is limited to 2, the one doing "ison" can do almost any other function: that of domestichos, canonarchos, ison, etc. In this case, when the melody "changes" to some new principal note (fthongos) the isokratis can change so as to "help" the teacher. This is the method that has remained in the "Patrarchal" ison of today, and which Stanitsas, and later on Vassilikos pushed to "extreme" = never-ending ison changes. When the choir was in complete form, the Protos canonarchos would change his recitation to the chord of the melodic line (for instance: in "ton pathon", ison is on Ni... the melody goes to Ga.. the Canonrchos does so, at "ta panta prosietai", and then comes back down to Ni for "tois prowkynousin en potho..). In choral form, the ison would stay on NI, in spite of the canonarchos' change of pitch. Yet, the canonarchos would do his ison at his pitch of recitation. This gives us a form of "double ison". In none of these situations does the canonarchos do a recitation on Pa or on Di = Pa (that is, he does not use a "dissonant" interval as to the basis, Ni in the first case, Ga in the second).
Now, when chanting alone with the teacher, on lines of plagal fourth concluding on PA , or on Di = Pa, Tsolakidis would change his ison. He explained to me that when the choirs were at their fullest "Pascha, Acathistos", they'd apply the above- mentioned "isokratima" tactics. On other days (about 95% of the situations), the Protocanarchos would learn to change just as modern Stantisas-type of choirs in very limited manner... for instance, a simple Pa on plagal fourth, with no Low Di, no Bou, ... in other words, the only other ison allowed, except for tri/tetraphonia of the canonarch's recitation, was the dissonant (as to the main ison) fthongos, with no double ison (the canonarch's recitation was also the ison). eg: Idou o Nymphios argon: Recitation on Ga = Ni... changes to Pa for "alla ananipson krazousa". Just about when started learning the neumes, he had to learn by heart where to change the recitation for Iakovos. And here, we add the other very important purpose of ison: it's an integration element = it is used to initiate novices to psaltiki, so that they may progressively "blend in" and participate.
"Ena einai to ison" does not come just from Tsolakidis, but from Iakovos... he would teach him to "change" ison in one or two places (and he even told me when Iakovos told him this... =when they were studying "ton Agion Pateron"), all in saying "kanonika, ena einai to ison" normally, one is the ison. the variable ison in a few positions, by the entire choir, and especially as concerns dissonant note as to the original ison is really to be considred as a concession. When he chanted "ton Nymphona sou", "makarios anir", etc, he would ask me to keep tha same ison, and not even to "chant along the lower than ison fthongos". Is pieces such as fourth stichiraric, ison = bou, and [co-chanting] "mazi" for descending lines. Quite honestly, this is one mode where it's hard to keep a constant Bou... the isokratis was always in advance on coming back to the BOU (the psaltis would not have to "imagine it), before the psaltis would go on chanting. third mode always GA, even if the melody would go beyond Ke.. or below Ga