If you're going to use autofocus, then one thing to consider is breathing -- i.e. the lens "zooming" when it changes focus. (This is nothing to do with zoom lenses specifically.) For example, I have the RF 50mm f/1.2, which is a superb lens, but using it for a piece to camera with AF, it was hopeless -- the slightest move by me made the background throb distractingly. You could mitigate this with a neutral background, like a plain wall, but that's boring, of course. So for any lens you're interested in, look for a review that talks about breathing.
Or you can use manual focus -- use a deep depth of field, and don't move too much.
Otheer than that... think about how close you want to be to the camera, because this determines the perspective you will get. (This is the ONLY THING that controls perspective.) A lot of people would like 6-10 feet (2-3 metres) for a natural-looking perspective, but I know I usually go closer due to space limitations. Once you've got that, pick a focal length that gives you a good field of view.
When you say you have the 18-45, and you're "not entirely sure that’s the best for me sitting at a desk", what do you mean? Do you mean that you can't fit yourself in frame? Because with an 18mm lens, you would need to be really close to the camera. personally I would have thought that you would want a longer lens, not wider. I have the RF 24-70 f/2.8, which has pretty well-controlled breathing, and that would be my choice -- albeit on a full-frame camera, but still, at the 24mm end it seems to me it should easily be wide enough.
Basically experiment with the lens you have and think about what the problem is. If you really can't fit yourself in the frame, then you need to move the camera back, because it is WAY too close. OTOH if you're tool small in the frame, even at 45mm, then you need a longer lens, not wider. (Though you could move the camera closer.)
