saṅkhāra = noun = essential condition; a thing conditioned, mental coefficients
saṅkhata = past participle of verb saṅkharoti = conditioned; prepared; produced by a cause
All of the translations, except I.B. Horner, have not used the translation of "conditioned thing", probably to ensure saṅkhāra is differentiated from saṅkhata.
The dictionary says:
saṅkhāra one of the most difficult terms in Buddhist metaphysics
The best translation for 'saṅkhārā' in 'sabbe saṅkhārā aniccā' will be the broadest translation.
For example, if we consider Bhikkhu Bodhi's translation of SN 22.90, it says:
When this was said, the elder bhikkhus said to the Venerable Channa: “Form, friend Channa, is impermanent, feeling is impermanent, perception is impermanent, volitional formations are impermanent, consciousness is impermanent. Form is nonself, feeling is nonself, perception is nonself, volitional formations are nonself, consciousness is nonself. All formations are impermanent; all phenomena are nonself.”
Above, Bhikkhu Bodhi has translated the 1st use of saṅkhārā as "volitional formations". Similarly, Sujato has translated the 1st use of saṅkhārā as "choices". The 2nd use of saṅkhārā, Bodhi & Sujato have translated as "formations" & "conditions". The different translations of the 2nd use of saṅkhārā is to highlight the 2nd use of saṅkhārā includes form, feelings, perceptions, mental formations & consciousness as being saṅkhārā. When SN 22.90 says 'sabbe saṅkhārā aniccā', it is saying form, feeling, perception, mental formations & consciousness are all saṅkhārā.
To conclude, none of the translations offered are inherently accurate as an explanatory principle because the word 'saṅkhārā' is too broad to be translated using one single English word. However, as already said, the best translation is the broadest, which is Sujato's & Ānandajoti's "conditions". This said, bear in mind, the word "conditions" here refers to both:
- prerequisite/causal agent conditions
- resultant conditions
Btw, saṅkhārā is never translated as 'thing' or 'phenomena'. The Pali for 'thing' or 'phenomena' is 'dhamma' or 'dhatu'.