Are any of the LDS gods without beginning or end, or are they all created at a certain point in time?

TLDR: As noted in comments your question(s) are in the final paragraph:

...Is that actually true, or have I misunderstood?

I would say you have misunderstood in part. You are correct that the LDS believe Jehovah/Jesus is the first begotten child of Elohim, but that doesn't remove the eternal nature of either.

Or do they say that Elohim is the eternal, uncreated, self-existent One?

Elohim is also eternal, less is known about his "beginning/creation" (this is in reference to the couplet: As man now is, God once was) as you reference it, but again He is still eternal. The only theology the LDS have on "self-existent" is that intelligences are/were. (see below D&C 93:29) One would probably be safe to say that as spirits are formed from intelligences, we all were "self-existent", though how that definition ties into our current phase is complicated.

The Church of Jesus Christ of Latter-day Saints believe all beings are eternal in nature. The phases/stages of one's eternal nature:

1. intelligences:

   the spirit element that existed before we were begotten as spirit children.

   D&C 93:29

   29 Man was also in the beginning with God. Intelligence, or the light of truth, was not created or made, neither indeed can be.

   Abraham 3:22

   22 Now the Lord had shown unto me, Abraham, the intelligences that were organized before the world was; and among all these there were many of the noble and great ones;

   _{The exact process/transition from intelligence to spirit is unspecified in LDS theology. (while intelligences probably fall under your definition of "self-existent" past this point "self-existent" probably doesn't apply-but in a sense it does)}

2. spirits

   That part of a living being that exists before mortal birth, dwells in the physical body during mortality, and exists after death as a separate being until the Resurrection....Each person on earth has an immortal spirit body in addition to a body of flesh and bone.

3. mortality

   Mortality refers to the time from birth until physical death.

4. immortality

   Immortality is to live forever as a resurrected being. Through the Atonement of Jesus Christ, everyone will receive this gift. Eternal life, or exaltation, is to live in God’s presence and to continue as families

5. Exaltation/godhood (continuation of 4)

   Through the Atonement of Jesus Christ, all people may “progress toward perfection and ultimately realize their divine destiny.”

So Jesus/Jehovah and Elohim are eternal, as are we all. How many times the process goes back (man becoming god, creating men) is unknown exactly (see this answer) and also isn't pertinent to one's salvation as we only have one God (see this answer). Measuring man's time vs God's time is complicated (see this answer)

On the couplet by Pres Lorenzo Snow (source in OP) “As man now is, God once was: As God now is, man may be.”, its important to remember little has been revealed about the first half of this couplet, and consequently little is taught (again I'd say this isn't pertinent to one's salvation). President Gordan B Hinckley said:

That gets into some pretty deep theology that we don't know very much about.

Q: So you're saying the church is still struggling to understand this?

A: Well, as God is, man may become. We believe in eternal progression. Very strongly. We believe that the glory of God is intelligence and whatever principle of intelligence we attain unto in this life, it will rise with us in the Resurrection.

I mention this in part to explain that the LDS have this deep theology (similar questions have been asked in a variety of ways, here are a few questions found on this SE), and there are details we don't know as they aren't pertinent to one's salvation (knowing how intelligences became spirits exactly won't change anything, knowing if God has a God also unimportant as we only have one) and probably because we can't handle the truth (as we all struggle with explaining current doctrine-as there are multiple denominations arguing over meaning over scripture AND this is a complicated subject for a variety of reasons)