The steps you've indicated should all be of some help, although it's difficult to say which ones will help most or how much help any of them will provide.

One additional thing that could reduce "echo," a problem in many mostly empty spaces, would be addition of "furniture" in the room, even to the point of a bit of "clutter." You might be able to find a cheap sofa and stuffed chair or two at one of the charity shops (vets, salvation army, etc.) and if they're not too nice you could locate them so nobody would be inclined to sit on them while you're working. Of course you won't want your office to look like you filled it with a fork lift, but not everything has to have an obvious function for what you're doing when the clients are there(?). You can pretend you do "other stuff" there too, even if you don't.

Even hard surfaced furniture, if spaced so that it doesn't just make a new wall, can help break up the acoustic space. If you need more than one filing cabinet, set them so there's at least a few inches between them so they aren't a single surface to reflect sound. The "cavity" between them should help break up the sound.

For musical acoustics, heavier wall hangings would probably be better, but for conversational noise the weight of any fabric coverings shouldn't make a lot of difference, as long as they're fairly "limp." A curtain that hangs an inch or two away from the wall will usually work better than if it's flat up against a hard backing surface. Even "Venitian blinds" (slatted) left a little open can provide some damping of echo effects, and might be better for sound control than solid sheet blinds or even heavy curtains, and could still let some light - and air - through the window.

First thoughts only. We'll see what others have to suggest.

John