There is no room to add a sliding or pocket door, so that limits the choice to bi-fold doors or bi-fold converted to center opening French doors, which are much less expensive than regular doors. These can be attached to open in the same direction as the original door, but because they are half as wide it's easier to get to stuff behind them.

You could also have someone change the original door to open in the opposite direction. This requires re-mortising the door and frame for the hinges, chiseling out a new hole behind the strike plate where the latch fits when the door is closed, and changing the stop trim on the jamb if needed. And of course, filling in the old mortises and matching the stain. The only other idea, but much less expensive, is a retractable dog gate.

It's called a DOG gate. Extra tall,

https://www.chewy.com/regalo-easy-step-extra-tall-walk/dp/163756?utm_source=google-product&utm_medium=cpc&utm_campaign=hg&utm_content=Regalo&utm_term=&gclid=EAIaIQobChMI2ayVqIrY8AIVYolbCh3mMQNGEAQYAyABEgIOZvD_BwE

We use this pet gate from DR to kitchen. Pressure mounted (no screws) and easy to open/close as you walk thru.

There is also the Scat Mat (probably other brands too). I guess you’d have to step over it too, or maybe after the dog learns not to go in there it could be turned off but left in place.