I don't think that this proclamation is "so in conflict with millenia of Jewish law and practice", because while it was permitted in principle, in practice it was very rare, and generally frowned upon. I think that the higher prevalence of Sephardic polygyny is also an "outside influence", from the Arab culture, and much less the "base condition" of Jews before the ban. I say this because of how few polygynous marriages we have record of, before the ban. This ban was very unlikely to have been met with an "Oh no, what will we do!?" type of response, simply because it affected very few people (even those already married). Had Rabbenu Gershom expected serious opposition, it is unlikely that the ban would have been made in the first place (basic meta-law - you don't create a decree, that the public can't bare). The only real "conflict" I see with the Law, is IF the ban was created in response to Christian pressure (whether direct or implied).

Do you want Israel to be a Jewish theocracy?

Not exactly. From my POV, the expression Jewish Theocracy is mostly redundant. As I've likely said before, Judaism takes a MUCH more holistic approach to life than probably any other religion/philosophy. Among other things, Jewish Law includes detailed instructions on how people should be governed. But the term "theocracy", is not really applicable in the sense that it is used in English. We are not talking about a Church-run government, but rather a country governed by religious law. If you go to my Perpetual Interview meme entry, and read the entire following sub-thread, I explain in some detail, how the ideal Jewish government is supposed to work, and what the "checks and balances" are. As I also said at the beginning of that comment, this "theocracy" can only possibly come-about by the explicit will of the general public, so again, this wouldn't be a coercive type of "religious dictatorship", but even so, I imagine you'll find the idea too restrictive.