Select all always appears if you have no text selected and never appears if you have some text selected. Insane UI decision by apple but that's how it is.
In editable text fields you can tap a word a few times and it'll select the whole paragraph, if that's any help.
What drives me insane though, is double tapping a word is supposed to select that word. But I think starting in iOS 18 it started selecting the word and a random amount of surrounding words, but only about half the time. I couldn't tell you what it could possibly be trying to do but it's maddening.
Which means you can't select all on text which isn't editable - insane!