suggest you get the type of cellular window shades that can be positioned bottom up and top down. That way you can screen off a view into the room from the porch but still have light coming in through the top (or the top and bottom at the same time). Check out hunter dougles duette and other types.

I wouldn't commit to built ins at this point. There is a lot of modular shelving that would be fine and less expensive.

When you open the front door you should have a double sided bookcase to act like an old school shadow box divider. Maybe even something like this... OR just a freestanding tall bookcase. with a more narrow opening into the room at the far end.

This is a flaw in your home and worth any money you need to pay to make it the cool feature it could be.

I think a nice lower built in with art above would be a great wall to see as you're entering your home. Do you need an office and desk? Or a completely enclosed space? You may just want to add two comfortable armchairs with a small round coffee table instead. I don't think it's necessary to close it off unless you really want a private, enclosed space. I'd live there awhile to see how you want the room to function before enclosing it in any way.

Since this space doesn't have any windows, it will be important to have really good lighting. You could do built-ins, or just some bookshelves, along with adding the desk and perhaps two chairs or a small loveseat. We have included some ideas for you below from various designs of ours:

Westhampton Beach Home for Family and Friends · More Info

Bespoke Interior Design: Home Office · More Info

Watersound Beach New Construction · More Info

Pied-à-Terre · More Info

Designer Showhouse · More Info

My home is sorta like this and still no idea what to do with flex room , what did you end up doing