I guess it would be pretty straightforward to create a questionnaire that would fill in a simple database with important metrics. Without knowing your age, weight, measurements, flexibility and so on, we're shooting in the dark, as you didn't write much in way of context.
The Triban RC520 is pretty crap, it's not just in your head.
Assuming you're young enough, either flexible or willing to get flexible enough (not that hard), i dont think you need to overthink it too much, the VB 268 will probably be life changing, and you dont need to spend more, especially if you order it black. To customize, you can get decals cut locally (or via aliexpress) for cheap, it's a good way to save 100 usd.
Given your budget, you should assume you'll be your own mechanic going fwd, because labour cost post inflation & now that bikes are fully integrated can double the cost of the bike in a few visits to the LBS... You'll also learn a ton.
Expect to start with all the spacers on the 268, because that decathlon bike has a VERY relaxed geometry. Then you may find yourself removing spacers over time, which will mechanically go faster on the flat. From this perspective, make sure to buy a cockpit that allows to have spacers above the stem, otherwise you'd have to cut the steerer every time, which is a disaster. It would also be pretty smart to get a bar & stem vs cockpit, because you may also want to change stems.
From a bike fit standpoint, given the uncertainty, you would benefit from a bike that is NOT fully integrated, to allow making changes more easily.
Changing anything on a fully integrated bike is absolutely miserable.
The best bike is a comfortable one that fits you.
