Salam aleykoum Tutti_frutti and wakas,
Thanks for your comments but I have given my arguments in my pdf, taking into account the composition of the text and the relations that its different elements have between them (as opposed to an atomistic reading).
Thus, according to this structural, thematic and lexical analysis, the syntagm "wa-ṣ-ṣalâtu-l-wusṭâ" then means word for word: "and the prayer of the right balance/wusṭâ**". That is to say, a balanced prayer; neither too long, because ostentation reveals insincerity, nor too short, because neglect denotes lack of seriousness.
=> the meta-theme of the Quranic passage (2:178...242) is based on the principle of equilibrium, i.e. the absence of excess (being between two extremes).
**The term wusṭâ is the feminine of awsaṭ: medium, middle, half, in the right balance/way, balanced, median.