Tom,
First let me say I think your photos are well done. There are a few simple changes I think you can make that might make some slight improvements, but you are starting in a very good place.
First though, let me just say, buying one "overall" lens like a 28-300 is ok if you are after convience. They are not as sharp as lenses that are smaller in increments or just a "telephoto" zoom. It is a long and boring explanation. It deals with lens aberation and the subsequent correction. Wide angle lenses require one kind of correction and telephoto requires another. "All in one lenses" are in general, a compromise. Edge sharpness is where you pay your price. There are a few exceptions, but they are very pricey. Having said that, use what you have but in the future maybe look to two lenses.
There are two major things to help your photos, first back up and use your zoom. In general, magnification has nothing to do with how close you are to the item. If you have to, even zoom in software. Backing up will accomplish two things. Depth of Field, or "focus" will grow, and become less critical and also less of the width of your background will show. Telephotos operate at a narrower angle. (This is a difficult thing to explain, but easily undertood when you see comparison photos so Google - Angle of acceptance of lenses).
The second major thing is to change your angle lower or lay your pen down. You just need to be more parallel to you pen. This will also improve your depth of field greatly. It will however force you to use a different syle of background. Set up a little "seamless" background. Pens are small enough an 8.5x11 sheet works fine.
Lastly, not to insult or be argumentative with anyone on this post, but going to manual is rarley a good answer, unless you really know what you are doing. As a camera store owner and professional photographer, I hear almost daily how people take their cameras off auto and use manual. I always ask how they determine their exposure, they generally respond they use the meter in the camera. I ask if they have metered a select area or used a spot meter or changed the exposure from what the meter originally said. Rarely does anyone do that. This gains nothing! You have merely disconnected the auto mechanism and have done the same thing the meter would have done in the first place. And ditto for Auto focus. Auto focus uses contrast to focus in "spots" those spots are clearly marked....pick a spot and put it on your pen. If that is not where you want it generally composed and you are on a tripod, turn the auto focus off at that point and compose it the way you want after that. Or you could use "focuslock"... look that up in your camera instructions. To manually focus an auto focus camera is not like focusing a manual focus camera. In a manual focus camera there is "aids" to help you determine the correct focus, in auto focus there is not. You are sort of looking at the overall scene and focusing.
Also, spot metering should be used by people that know what they are doing. Camera meter systems, oversimplified, seek and make general assumptions. They think everything is 18% gray, so if you spot meter off a black or white section, it's reading is skewed. In the world of portrait and commerical photography we use only incident handheld meters. You can accomplish a similar thing by buying a "gray card" at your local photo shop and putting it where your pen is going to be THEN going to manual and using that reading for your exposure. Make sure that you only have the card in the viewfinder, nothing extra. Your camera may not operate correctly because it will not be able to focus, so take it off autofocus briefly, but make sure to put it back!!
This is the world of photography according to "moke". There are tons of ways to do anything in this business, so no absolutes, but this is what I have preached for a long time.
Feel free to PM me anytime, and I hope I do not appear too confident. This is merely my opinion.