I know this isn't what you want to hear, but don't spend on upgrades. Really, new tiagra shifts pretty nice, even sora. You most likely won't notice that much of a difference and may be disappointed. As said before, the biggest thing that will make a difference in your case is new wheels. The fork you posted still has an aluminum steer tube, and a carbon seat post won't make a difference you can "feel".

I was in your spot this year. I sold my bike and bought a new one. That is by far the best bang for your buck way to go. Just ride your bike now until you can buy a new one and sell your old one and offset the cost a little. Or just keep it as a spare.