how can anyone advise anything by going only on your description? You haven't given a floor plan, dimensions or size or placement of any rooms. And the most important, your budget.

if you want an ensuite, then obviously you're going to have move into the other room, borrow from an adjoining room, or build onto the master. Building on is your best bet because you don't lose any of the other bedrooms. It's also the most expensive.

borrowing closet space or other space from other parts of the house is another option. but since you haven't posted a floorpan, I don't know.

Kids, step away from the phone. Read your questions. Ask yourself: If
I don't know ME, I don't know my house, if I have not seen the thing
about which I have a question...... could I begin to answer my question?

Unless you can answer these questions, set that phone down, do not
hit that little green "submit" button. Here is some string to tie your
hands to your couch arms. Or....... expect no help at all.

Really that crystal ball gets quite a workout sometimes and then the next time we need to read a book.