Find me the line in Deuteronomy 20 saying any of this is good.

Also as I and others have said in other comments

1. this isn't exceptional for bronze age and iron age near eastern warfare; the weird part about biblical god is that there are written and codified limits to the destruction and plundering;

2. Genesis through 2 Chronicles are 'historical' books and not necessarily endorsements of the behaviour or laws depicted, as god can and quite explicitly does change his mind (e.g. Jeremiah and the Potter);

3. this behaviour (the killing, expansion of Israel and forced labour) is explicitly cited as one reason for Israel's destruction in Isaiah and Jeremiah;

4. Jeremiah explicitly says that the "lying pens of the scribe have handled it [the holy texts] falsely" and we know from linguistic and historical/archaeological research that the Pentateuch (the first five books) are the most heavily edited of all the books especially in terms of and around the sections of laws;

5. Deuteronomy is known to be one of the later and most priestly of edits;

So all in all I don't think Deuteronomy 20 is the gotcha you think it is (it is also bad priestly fanfiction to justify bad kingly fanfiction, but extratextual debunking is less fun)