I have a Decathlon Triban RC500 with all stock components which I'm quite happy with. Since I have cash to burn (actually not, this question is hypothetical :)) what component should I upgrade first to make it faster/lighter? What would give me most bang for my bucks?
For higher performance (speed) the simple answer is better wheels and tires. Deeper dish aero wheels are definitely going to be faster -BUT- will be more of a handful in crosswinds. Light weight deep dish rims mean carbon fiber and lots of $$$. Also note that rims are trending wider as this makes for a more aero shape and it gives the tire a stiffer sidewall.
A set of high end tires (Conti 5000's or comparable) are going to be more fragile than the stock tires and they'll wear out faster simply because good race tires have less rubber to keep the weight down. Interestingly a wider tire will roll faster over coarse pavement, but the optimal tire width for aerodynamics is 5% narrower than the rim. So for a rim with a 26mm external width you'd want a tire which is about 25mm wide.
Once you've upgraded wheels & tires then other component upgrades will have a smaller incremental reduction in weight.
For bikes in this price range one can often take off a good bit of weight by upgrading the following. BUT to know for sure you'll need to know the weight of your current parts:
Crankset & bottom bracket
Cassette - but you may have to jump to a Dura-Ace level to see a significant difference in weight.
Seatpost is often an inexpensive no-name part. Changing from aluminum to carbon (IMO) made a far greater change in ride than I expected.
Carbon handlebars, like a carbon seatpost, will definitely dampen/absorb some of the road vibration. You might save some weight here as well.
Saddle: the stock saddle is usually on the heavier side. There are lots of race saddles well under 200 gm. Check out Craigslist and other sources for used saddles as no one saddle fits everyone so there are lots of people getting rid of saddles which didn't work out.
Things which cost A LOT but don't generally reduce much weight: - New shift/brake levers - Brake calipers - Going from 10spd to 11spd is really expensive and it's usually much more cost effective to simply get a different bike.
Bottom line - you can drop A LOT of money in upgrading stuff.
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.