There are a number of options...
Wish
Let's go ahead and get this one out of the way. I'm not talking about using Wish to just Wish someone back from the dead, because that's a "strenuous use of the spell" which would (per your opinion on Reincarnate, count as a Cost), simply use Wish's ability to
duplicate any other spell of 8th level or lower. You don't need to meet any requirements in that spell, including costly components. The spell simply takes effect.
So, Wish for the 7th level spell Resurrection, and they're alive at no cost beyond a 9th level Spell Slot. Alternatively, you may pre-empt the risk of death by Wishing for Clone spells for the whole party.
True Polymorph
You covered this in your question, Turn into a creature with component-free access to resurrection magic, such as a Planetar.
Divine Intervention
Like wish, this ability can do basically anything as long as your DM goes for it. Until you are 20th level, the odds of it actually working are fairly slim--but it offers a cost-free (apart from the cooldown) way to raise the dead.
If everybody in the party takes a 3 level multiclass Dip into barbarian and takes the Path of the Zealot subclass, they can acquire the following Class Feature
Warrior of the Gods
At 3rd level, your soul is marked for endless battle. If a spell, such as raise dead, has the sole effect of restoring you to life (but not undeath), the caster doesn’t need material components to cast the spell on you.
This grants the following option:
The rod has 5 charges. While you hold it, you can use an action to cast one of the following spells from it: heal (expends 1 charge) or resurrection (expends 5 charges).
Spells cast through magic items do not consume material components (see here) This is, however, an edge case because...
The rod regains 1 expended charge daily at dawn. If the rod is reduced to 0 charges, roll a d20. On a 1, the rod disappears in a burst of radiance.
It can only be used to Resurrect once every 5 days, and has a 1/20 chance to vanish.
Scroll of [Rez Spell]
Scrolls require the expenditure of their costly material components when they are created. As long as someone else created the resurrection spell and the party acquired it for "free" in the course of adventuring, it's a Free Resurrection. The same applies to Spell Gems.
Artifact with the proper Major Beneficial Property
One of the possible Major Beneficial Properties of an Artifact is...
While attuned to the artifact, you can use an action to cast one 4th-level spell (chosen by the DM) from it. After you cast the spell, roll a d6. On a roll of 1–5, you can’t cast it again until the next dawn.
There are versions of this property for spells up through 7th level, and there are several life-restoring spells that would fit into that range.
Convince something to do it
If you can locate a creature capable of raising the dead without components, then social skills may be able to persuade them to do it. For example, casting Plane Shift (or Gate) to go visit somewhere with Celestials and Persuading, Deceiving, or Intimidating them into doing it.
Mind Effecting Magic
If you can locate a creature capable of raising the dead without components, there are all kinds of mind effecting spells that can "convince" them to help. Suggestion or Charm Monster on the lower end, Dominate Monster on the higher end. You will have to work fast with these, as Charm Monster and Dominate Monster have a Duration that's the same as the casting time of most Resurrection Magic.
