Wooderson

The type of riding you describe is endurance riding, so I think it makes sense to focus on endurance bikes. I just replaced my Orbea Avant (endurance) with a Giant Defy Advanced 2 (endurance), and it's an amazing bike that is incredibly comfortable on the stock 32mm tires. I've already put 700 miles on it in the first month, and feel markedly less fatigued than if I did the same on my Orbea (25mm tires). The new Defy is a full 2lbs heavier than my Orbea, too.

It is possible to make an endurance bike aggressive, and it's possible to make a race bike tall and upright (I actually see that a lot). So there is some crossover; it's not as black and white as many make it out to be.

I would only consider disc bikes at this stage. 95% of road bikes sold in three years will be disc, and buying disc now is a form of future-proofing. It will affect resale value and future upgrade options. I saw this happen in the mountain bike space about 10 years ago.

All of the big manufacturers make pretty much the same bikes at at their respective group levels / pricepoints (within a category, of course). Sure, some have "zert inserts" or "d-fuse seatposts", but I wouldn't rank those marketing-driven features higher than your test ride experience. There are some subtle differences that matter, like my Defy 2 is full 105, except they cheaped-out by putting a non-group crankset on it. The 105 (hollowtech) cranks would have saved 40 grams. That's partly how Giant priced it $400 less than the Domane (wheels are the other part).

So I voted for the Defy, because I just went through this process, and really love my new bike. But I also really like the Domane, Roubaix, and Synapse. You can't go wrong with any of those. Buy the one you feel most comfortable on, and just like the best. You're going to be the one riding it.

Edit to address a couple of your specific questions: Giant is on par with the other big manufacturers, your friends's perspective is outdated and wrong. I'd guess that the seat cap issues you've read about on the Roubaix are isolated, I haven't heard the same.. either way, I wouldn't think it would be a huge deal, the bike is warrantied, and your dealer will take care of you if problems like this occur, so don't let that sway you too much. BTW, the Roubaix was on my short list, too. Brakes are why I bought the Defy, the Defy has 105 hydraulic brifters and calipers, the Roubaix has mechanical brifters and Tektro mechanical disc brakes. The Shimano hydraulic brakes are materially better, IMO.