What exactly is your locale; what exactly do you choose in Text Import dialog; and what is the exact problem you see after that - with screenshots? Up to now, you provide “analysis”, but no description of the problem (I do specifically this, and I get that).
You wrote several times: “format”. Once it even was “YMD” display format - and that seems to tell that you think, that in the import dialog, you ignore the words “Column type”, and imagine that you set up there some “display” settings. No you do not. In the import dialog, you only tell Calc how to import - meaning “detect parts of the date in the text of the CSV”; and all formatting will happen only after the import, in the normal UI. If you choose the correct settings, the dates will be detected correctly: the ISO dates are always detected, when you check the “Detect special numbers” checkbox in the dialog; and the non-ISO dates like “31/10/2025” require the “DMY” in the respective column (not because you want them to display that way, but because you want Calc to know, that in those dates in the CSV, the first part is day, then month, then year).
With those settings, you get a column with differently formatted dates, but all are dates, and will sort correctly (as dates, not as text); and you would also be able to format them uniformly - by configuring cell format (or better style).
