Depends if you hold your breath
The player's handbook p.183 gives rules for suffocating. In brief a creature holding its breath can last for minutes but a creature that runs out of breath or is choking lasts a number of rounds equal to their constitution modifier before dropping to 0 hp.
The question then is whether a PC is holding their breath when being suffocated by, say, a darkmantle. The darkmantle monster entry says a creature engulfed is, "unable to breath", but gives us no further information. This seems to be up to the DM to rule if a given character was able to begin holding their breath, and so lasts minutes, or if the suffocation begins suddenly and they can only last a few rounds.
Personally I'd go with effects such as this happening too suddenly to hold your breath since it keeps the effects relevant in an average battle.
RAW, you can hold your breath because Jeremy Crawford says so
If we're counting JC tweets as RAW than all that matters is JC says you can. As you explained in your question this means that the suffocation part of fighting a darkmantle is unlikely to ever come up in a normal battle, and I'd say this takes away what makes the darkmantle an interesting monster. Just another strange part of 5e RAW.
Spell choices to do what you want are limited, so I'm including Wondrous Items in my answer. I hope you don't mind. Prices are purchase, not craft.
For Poison type Damage
You could consider Ointment, Restorative; 4000g for 5 uses. The spell it uses is Neutralize Poison (Cleric 4) which can prevent worsening Damage but not heal it.
Greater Neutralize Poison(Cleric 6) counters Damage from poison and is a Standard Action.
Periapt of Proof against Poison sets you back a hefty 27,000 ea but is a permanent solution.
For Negative Energy (undead etc)
Your GM may or may not allow Death Ward (Cleric, etc 4th-5th level) to prevent it. It works on negative levels but it's unclear whether ability Damage from undead creatures falls under Death Ward's protection against "any negative energy effects". I think most GM's allow it to work, but I'd ask instead of assuming.
More Generally
Contingency (Wizard/Sorc 6) with Restoration or similar 'when an Ability falls below X' or just when 'x Ability Damages accrue'.
It's not magic but Barbarians can take Renewed Vitality to help themself.
Ring of Inner Fortitude (16,000/42,000/66,000 ea) can mitigate all forms of Damage (and as a bonus Drain) by 2(1), 4(2), or 6(3) until you have time to heal up. If you take 2 Dex and 2 Con Damage, you would effectively not lose anything, the 2(4, 6) applies to each Ability separately.
Stalwart Resolve (Cleric 2) allows you to ignore all Damage to 1 Ability for 1rd/level of the Cleric. It might get you through a fight, at least. Bonus, you could buy/make scrolls or wands of it. Takes a Standard Action to counter one type of Damage, though.
Alternately, refocus on preventing the Damage. Buffing the party's Dexterity and Constitution will help you get hit less and succeed Fort saves more, preventing ability Damage/Drain.
Best Answer
There is a magic item that will permit breathing in such an environment:
You can rule that "any environment" includes an airless void.
A problem with this approach is that you'd need to equip each PC with such a necklace.
This item could be re-skinned as a Ring of Adaptation using your DM's discretion.
Re-skinning a current spell/item (Water Breathing)
You can re-skin Water Breathing (Druid/Sorcerer/Wizard 3rd level) to fit this situation. The spell is within your party's power level. It allows a character (via magic) to breath in an environment that normally does not contain air/oxygen. (We won't get into dissolved oxygen in water, this isn't a physics class). It's not too far of a reach to repurpose this spell for the environment you present to the characters (torches don't work underwater either). Changing this spell slightly could require some successful Knowledge and/or Arcana checks, research, or other downtime activity. (In the case of the Druid, you can implement this via prayer/ritual with whatever power/deity the druid serves).
Alternatively, have available for trade, or have the party find as a set up to this adventure
scrolls with this adapted version of the water breathing spell
or
a wand with X charges of this adapted version of this spell.
A wand of fireballs (3rd level spell) is rare, this wand would be rare as well.
You indicate that this is an extended adventure. The wand might be the best fit if you treat it like the wand of fireballs and have it contain limited charges and recharge at dawn each day. Absent the wand, one of the casters with the spell burns a third level spell each day to keep the party alive for the adventure. (Keep that spell caster alive or TPK is imminent!)
The wand may be the simplest way to do this: it precludes research/downtime.
Another item can fit into this idea: re-skinning a ring's effect slightly can yield a Ring of Water Breathing.
If you want to borrow an idea/item from 1e AD&D, the Iridescent Ioun stone in that edition "sustains a person without air" (DMG 1e p 147). Ioun stones are in this edition, but I did not find that particular one in the book.