For me, the main difference is groupset and tyres, anything less than a shimano 105 can get annoying, and on my 1st bike I was averaging a puncture every 50 miles or so.
I think either decathlon bike, the ali or all carbon one, will be ok on both counts.
The one Iron-Blisters recommends looks to fit the bill, and if you can get to a store, they should do you a fitting, but, as before, if you've got a good local bike shop, they may be more expert at setting up
difficult to recommend a particular brand/model as its very subjective.
I started with a 500quid Orbea, which I was very happy with, until it got nicked. Upgraded to a 1500 Orbea with Ultegra groupset and difference was immense, so much quicker. But, I can be beaten easily by my son-in-law riding my 400quid Halfords hybrid, so its not all about the bike.
It makes a bit of sense to go for a reasonable price bike to start, too cheap gives you reliability problems, gears don't work as well, but going for the expensive option right away can lead to bit of "all the gear, no idea" syndrome (and very expensive repairs if you drop it).
or something similar, but a visit to a local bike shop might be better.
Its ultra important to get set up on the bike, right size frame, seat height/position, handlebar height etc. makes a huge difference.
If, after a year or so,you want to upgrade, then, like lots of us, you may end up with a bike for training, or used with a turbo trainer over winter and your brand new race bike, which by then you will know a lot more about what you like or need.