What would you upgrade first on a road bike?
Honestly?  Nothing.   Assuming your bike functions correctly and is not yet worn out?  I'd not change anything on the bike mechanically.

Perhaps think about comfort, what would allow you to spend longer on the bike, or to go harder?  What hurts at the end of a long ride, and how can you improve that?

Changing Contact points can help, so better gloves/bartape.  Perhaps shoes and cleated pedals, but there's nothing wrong with flats (its really hard to walk around the shops in cleats)

If you do night riding, then some decent lights can be wise, two front and two rear (so a primary and a backup in each direction)

Clothes make the ride more enjoyable too.  Close fitting fabric is faster than loose baggy clothing, and it often weighs less too.  So a riding top and tights/leg warmers might be of interest.

As for progress - if you're data-driven then using a service like Strava or mapmyride or similar can help you know that you're improving.  They offer free levels, and more for a subscription.  These services may work on a standard smart phone, or you might choose a fancy bike computer.



But seriously, wear parts of your bike out before upgrading.  That's fiscally responsible.  

When your tires wear to the point they get a lot more punctures, then replace them with something nice like GP5000 or similar race tyres.  But not earlier.