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By "theistic God", I mean a God who is omnipotent and omnibenevolent and expressed himself through the Tanakh, or at least as powerful as say, Apollo was believed to be in ancient Greek religion.

Different sources seem to be saying different things. Wikipedia says yes, while ReformJudaism.org says not really:

• 
  Wikipedia: "In regard to God, while some voices among the spiritual leadership approached religious and even secular humanism – a tendency that grew increasingly from the mid-20th century, both among clergy and constituents, leading to broader, dimmer definitions of the concept – the movement had always officially maintained a theistic stance, affirming the belief in a personal God."


• 
  https://reformjudaism.org/blog/2018/01/19/do-you-have-believe-god-be-jew: "When people tell me that they don’t believe in God – either because they have seen no empirical evidence that God exists or because they can’t rationally accept the God of the Bible and of the medieval rabbis – I understand completely. I don’t believe in that God either. [...] God is the mystery within us – inside every soul, in the love that inspires generosity and compassion."


• 
  https://reformjudaism.org/practice/ask-rabbi/can-reform-jew-believe-torah-word-god: "In fact, I know of one Reform Rabbi who believes that Torah was revealed at Sinai [...] but that doesn't disqualify him as a Reform Rabbi! We're a big-tent religion, and everyone agreeing on everything isn't the point, and it's never going to happen, anyway. [...] Many Reform Jews have a deep belief in a God that is not literal or theistic, but rather is more mystical or spiritual."


• 
  https://www.timesofisrael.com/9-things-to-know-about-reform-jews/: "About 29 percent of Reform Jews say they believe in God with absolute certainty, compared to 41 percent of Conservatives and 89 percent of Orthodox."
  
  
  • Note that the article doesn't say how the respondents define God.
  
  

By contrast, Reconstructionist Judaism is quite clear that it does not believe in a theistic God.

I'd like recent official statements from organizations representing Reform Judaism, even if it's just that Reform Judaism has no official position on this, and/or a survey of Reform Jews that allows for nuanced perspectives of what God means to them.

Do Jewish people believe in God? is a related question but does not specify Reform Judaism (and I think by default questions on this site are assumed to be about the Orthodox perspective).

As in any group of Jews, there is variation in what people believe. The blog posts (which are not official positions of the Reform movement) bear this out, as do the survey results you reported.

The Reform movement rejected a membership application from a congregation that billed itself as "secular humanist". At the individual level, a responsum addressing atheist and agnostic conversion candidates concludes that an atheist cannot be accepted as a convert, though for an agnostic there is room for further evaluation:

The most recent CCAR rabbinic manual, published in 1988, maintains the tradition of questioning the prospective convert’s belief in God. The first question asked is: “Do you choose to enter the eternal covenant between God and the people Israel and to become a Jew of your own free will?”10 The implication is clear. To become a Jew, Reform Judaism demands that the convert affirm belief in God and the unique bond between God and the Jewish people.

It must be emphasized that the declaration of faith does not demand that the ger /gitoret adhere to a particular God concept, but simply that he/she be able to affirm the reality of God in our religious experience. [...]

Consequently, if, in the opinion of the attending rabbis, she is an atheist, then the position of the Responsa Committee is well known. She is not to be accepted. However, if she is, as an agnostic, simply unsure or confused, then she should be carefully instructed and introduced to the diverse theological teachings that enrich our faith. Let her be taught that we are not so arrogant as to claim to know all about God, but neither is our faith so unsure that we can fathom life without God.

The responsum acknowledges that atheist Reform Jews exist, and seems to be saying that there's not much we can do about that. (However, standards for conversion are higher.) In general, the Reform movement focuses more on what you do than what you believe, at the personal level. A known atheist would not be welcome to lead the community in prayer, for example, but in my experience, nobody investigates the private beliefs of the people who show up for minyan.

The Central Conference of American Rabbis (CCAR) is the Reform Rabbinic leadership organization. They came up with a number of "platforms" that are "documents capturing the state of Reform Jewish thought at different key moments in our history".

In general they don't affirm a specific perspective on God, His omnipotence and benevolence and seem to accept different views, just as you have documented.

Specifically they stated in 1937 (here)

The heart of Judaism and its chief contribution to religion is the
doctrine of the One, living God, who rules the world through law and
love. In Him all existence has its creative source and mankind its
ideal of conduct. Though transcending time and space, He is the
indwelling Presence of the world. We worship Him as the Lord of the
universe and as our merciful Father.

Their 1976 statement on the topic reads

The affirmation of God has always been essential to our people’s will
to survive. In our struggle through the centuries to preserve our
faith we have experienced and conceived of God in many ways. The
trials of our own time and the challenges of modern culture have made
steady belief and clear understanding difficult for some.
Nevertheless, we ground our lives, personally and communally, on God’s
reality and remain open to new experiences and conceptions of the
Divine. Amid the mystery we call life, we affirm that human beings,
created in God’s image, share in God’s eternality despite the mystery
we call death.

The "Statement of Principles for Reform Judaism" adopted in Pittsburgh in 1999 mentions

We affirm the reality and oneness of God, even as we may differ in our
understanding of the Divine presence.