Are 'elements' defined as non-suffering?

You are mistaking Saṅkhārā for concepts. Concepts are perceived as stable and satisfying. Saṅkhārā means conditioned phenomena. That refers to physical elements and the rest of the five aggregates. All saṅkhārās come under the first noble truth for being deficient, impermanent, unpredictable & conditioned. But there will be no mental suffering if you do not cling to them. Think of it as a fire. You don't get burnt as long as you stay away from it. But that doesn't mean the fire isn't hot.

There's no non-suffering element except for Nibbana.

Absolute realities fall into citta, cetasika, rupa, nibbana out of which the Rupa is elements or materiality. [See: The Abhidhamma in Practice by N.K.G. Mendis] Nibbana is also sometimes considered an element.

Everything which is conditioned is constructed by materiality or elements which are:

• earth or solidity
• fire or heat
• water or cohesion
• air or movement
• other secondary elements

What ever made of elements is unsatisfactory and suffering as it is not in one's control or not permanent (hence you will have to part with pleasant forms causing unsatisfactoriness).

The element or part that causes suffering in this is the mental reaction to your experience. When you do not react to it with craving, aversion of ignorance then there is no suffering. You will be at peace or Nirvana if you do not react with craving. As Nirvana is sometimes also referred as an element (Nibbānadhātu) it could be this which can be considered as an non suffering element.

Citta, cetasika are also conditioned by they come into being as part of the Nama-Rupa process, hence they cannot exist without materiality except for in the immaterial realms.

Nibbānadhātu

Nibbānadhātu:—“the state of Nibbāna”. S.V, 8; A.II, 120; IV, 202; J.I, 55; It.38 (dve: see under Nibbāna); Miln.312. Also in the foll. connections: amata° It.62; bhū° the verbal root bhū DA.I, 229; ṭhapitāya dhātuyā “while the bodily element, i.e. vitality lasts” Miln.125; vaṇṇa° form, beauty S.I, 131; Pv.I, 31. In these cases it is so far weakened in meaning, that it simply corresponds to E. abstr. suffix —hood or —ity (cp. °hood=origin. “form”: see ketu), so perhaps in Nibbāna°=Nibbāna-dom. Cp. dhātuka.

Source: Sutta: The Pali Text Society's Pali-English Dictionary nibbānadhātu : (f.) the sphere of nibbāna.

Source: BuddhaSasana: Concise Pali-English Dictionary

The Ultimate Realities

The Abhidhamma deals with realities existing in an ultimate sense, called in Pali paramattha dhammaa. There are four such realities:

Citta, mind or consciousness, defined as that which knows or experiences an object. Citta occurs as distinct momentary states of consciousness.

Cetasikas, the mental factors that arise and occur along with the cittas.

Ruupa, physical phenomena, or material form.

Nibbaana.

Citta, the cetasikas, and ruupa are conditioned realities. They arise because of conditions and disappear when their conditions cease to sustain them. Therefore they are impermanent. Nibbaana is an unconditioned reality. It does not arise and therefore does not fall away. These four realities can be experienced regardless of what name we give them. Any other thing — be it within ourselves or without, past, present, or future, coarse or subtle, low or lofty, far or near — is a concept and not an ultimate reality.

Citta, cetasikas, and nibbaana are also called naama. The two conditioned naamas, citta and cetasikas, together with ruupa make up naama-ruupa, the psycho-physical organism. Each of us, in the ultimate sense, is a naama-ruupa, a compound of mental and material phenomena, and nothing more. Apart from these three realities that go to form the naama-ruupa compound there is no ego, self, or soul. The naama part of the compound is what experiences an object. The ruupa part does not experience anything. When the body is injured it is not the body, which is ruupa, that feels the pain, but naama, the mental side. When we are hungry it is not the stomach that feels the hunger but again the naama. However, naama cannot eat the food to ease the hunger. The naama, the mind and its factors, makes the ruupa, the body, ingest the food. Thus neither the naama nor the ruupa has any efficient power of its own. One is dependent on the other; one supports the other. Both naama and ruupa arise because of conditions and perish immediately, and this is happening every moment of our lives. By studying and experiencing these realities we will get insight into: (1) what we truly are; (2) what we find around us; (3) how and why we react to what is within and around us; and (4) what we should aspire to reach as a spiritual goal.

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Ruupa

The third reality or paramattha dhamma is ruupa, matter or material form. In its analysis of matter the Abhidhamma recognizes twenty-eight kinds of material phenomena. Four of these are called primary, twenty-four secondary. The secondary kinds are dependant on the primary.

The four primary elements (cattaari mahaa bhuutaani)

These are metaphorically referred to under their ancient names but signify distinct properties of matter:

The Earth element (pa.thavi dhaatu) = solidity

The Water element (aapo dhaatu) = adhesion

The Fire element (tejo dhaatu) = heat

The Wind element (vaayo dhaatu) = motion

There is no unit of matter that does not contain these four elements in varying proportions. The preponderance of one element over the other three gives the material object its main characteristic.

The Abhidhamma in Practice by N.K.G. Mendis

MN 115 is a comprehensive discourse on elements (dhatu).

In reality, none of the many elements described in MN 115 are intrinsically 'suffering' because they, as mere elements, are not yet mixed with attachment (upadana).

Thus, MN 115 states:

When, Ānanda, a bhikkhu is skilled in the elements... in that way he can be called a wise man and an inquirer.

The 1st noble truth summarises all suffering as 'attachment' (upadana) and the 2nd noble truth explains the various cravings (tanha) do not give rise (samudhaya) to suffering (dukkha) unless craving creates new becoming (bhava), which is simply more solidified attachment (upadana).

To quote the 2nd noble truth from SN 56.11:

And this, monks, is the noble truth of the origination of stress: the craving that makes for further becoming — accompanied by passion & delight, relishing now here & now there — i.e., craving for sensual pleasure, craving for becoming, craving for non-becoming.

The teaching of the 'elements' ('dhatu') is the same as 'sunnata' ('emptiness') therefore any clear discernment of 'elements' quenches suffering because 'elements' are discerned rather than 'self'.

Again, to quote, this time from MN 62:

Rahula... there are these five elements, Rahula. Which five? The earth element, the water element, the fire element, the wind element & the space element... Now both the internal elements & the external elements are simply elements. And that should be seen as it actually is present with right discernment: 'This is not mine, this is not me, this is not my self.' When one sees it thus as it actually is present with right discernment, one becomes disenchanted with the elements and makes craving for the elements fade from the mind.

The phrase: "sabbe saṅkhārā dukkhā" is unrelated to this answer posted in the question because the word 'dukkhā' in this phrase does not mean 'suffering'. Instead, it means 'unsatisfactoriness', i.e., the incapacity of saṅkhārā (impermanent conditioned things) to bring lasting true happiness.

This is clearly explained, also in MN 115, as follows:

Here, Ānanda, a bhikkhu understands: ‘It is impossible, it cannot happen that a person possessing right view could treat any formation as permanent ― there is no such possibility.’ And he understands: ‘It is possible that an ordinary person might treat some formation as permanent ― there is such a possibility.’ He understands: ‘It is impossible, it cannot happen that a person possessing right view could treat any formation as pleasurable ― there is no such possibility.’ And he understands: ‘It is possible that an ordinary person might treat some formation as pleasurable ― there is such a possibility.’ He understands: ‘It is impossible, it cannot happen that a person possessing right view could treat anything as self ― there is no such possibility.’ And he understands: ‘It is possible that an ordinary person might treat something as self ― there is such a possibility.’

Therefore for example, inanimate objects, such as rocks, trees or stars in the sky, are 'unsatisfactory'; they cannot bring or be relied on for true pleasure; but they are not 'suffering'.

To avoid doubt, Dhammapada 278 explains the experience of "sabbe saṅkhārā dukkhā" results in liberation rather than suffering:

"All conditioned things are unsatisfactory" — when one sees this with wisdom, one turns away from suffering. This is the path to purification.

Dhammapada 278