the sacking is hard coded just to horde nations - and only before they have settled down and occupied a city. You can make the saxons a horde nation and start them on the map without a city (or with one city they can leave to form a horde) if you want to do that see: https://forums.totalwar.org/vb/showthread.php?t=58385

you could possibly also simulate something with scripting so they get more money when they take a settlement, kill more population and maybe damage buildings (not sure on that), scripting is a bit beyond poor Mak though so you need someone else for advice on that.

and welcome to the forum