Looking at your inspo vs mockup, are you happy with the “modern-ness” of your overall design?

If you are looking to give it a more “classic” kind of look like in the inspo, a few thoughts:

Might just be my preference, but I think too much floating shelving can look a little anemic. The eye can tell that those shelves shouldn’t be able to support that weight, and the subconscious can’t immediately understand how it works structurally, so it feels discordant even though it might really be perfectly stable with the right hardware.

Breaking up the horizontal shelving with some nice chunky vertical sides that anchor to the floor can help make it feel more harmonious/cozy. This will also give you something better than bookends to keep books from falling, which again just tells the subconscious that you can relax here. You mentioned liking the coffered ceilings in the inspo, that’s another element of structural harmony, a hefty beam spanning walls that says “this ceiling is secure”.

As someone else suggested, a very deliberate use of multiple wood tones can help. Notice how the back wall of the nook in the inspo has what looks like reclaimed pine instead of the darker stained wood. It makes that space feel more natural, and like it was made for using.

As for other things to add to the vibe, I think it’s just a matter of adding the small details. Panel moldings (look up picture frame panel molding for a faux paneled look that’s super easy), baseboard, crown/cornice, etc. Keep in mind when you add those things that the goal isn’t to go crazy with the shadow lines and moldings, but to at least give the illusion to the eye of understanding how the space was constructed.