What is the ruling on saying "Allahu Akbar" aloud in unison after congregational prayers during the days of Tashreeq? And which is more preferable: the customary post-prayer glorifications or the "takbir"?

There is no harm in communal takbir (pronouncing "Allahu Akbar") in a single voice during the days of Tashreeq. It is preferable to combine takbir with tasbih (pronouncing "Subhanallah"). However, if one must be prioritized, takbir should be given precedence over tasbih and the recitation of Ayatul-Kursi.