Layout Options > Page Setup > Page Size > Orientation - set to Landscape.
I’m not sure that locking systems (or their positions) is helpful, here, as you’ll presumably want Dorico to shuffle bars around in order to fill out systems that will be wider in landscape than in their current (portrait) form.
I generally just add an entirely separate landscape layout for this purpose so it doesn’t disturb any other layouts already present. That way systems will flow naturally according to Dorico’s routines according to the layout parameters you have set, and setting system breaks need only be set where you wish to override the existing casting off.
I also copy down the settings I use for the landscape format so I can use it agai in subsequent files as needed.