@TB
Frame size is very important on a road bike, the top tube is the one that's best to look at as this determines how far you are going to be stretched out.
You can always small frame bigger (longer stem, seat post set back etc) but you can't make a big frame smaller if you get my drift. A jump from 54 to 56 can be significant, 54 to 58 is huge. Do you know your measurements and have an idea of frame size you need? If not start
here.
Road bike are inherently uncomfortable, it's the nature of the beast. Running over a pebble can feel like running over a boulder hence getting the bike the most comfortable to start with being important. You will however get used to it so don't let that put you off at all.
The bike you linked is more of a time trial bike and honestly avoid for a first bike, re comfort or lack of it. Multiply for a time trial bike. On to the components yep Ultegra is quality stuff, the Shimano running order is.
Dura Ace
Ultegra
105
Tiagra
Sora
Then the stuff you would find on a Walmart bike.
Keep in mind Shimano are the masters of trickle down tech so last generation Dura Ace is like current Ultegra, Ultegra like 105 etc etc.
The components on the listed bike are about 3 generations old Ultegra so around the quality of today's Sora, the current Sora is probably a bit better tbh. Not that there is anything wrong with that until you hit the Walmart stuff all Shimano's gear is of pretty good quality. 👍
The price of bikes falls quicker than an out of control satellite re-entering the atmosphere so keep looking and you'll soon get a very good bike at a great price. Lot's out there of from people who have great intentions of riding and buying a high end bike only to ride it half a dozen times. You don't need to spend a fortune to get started, that can come later.
Don't hesitate to yell out if you have any questions, I'll do my best to help and answer as honestly as possible.
Give me 6-12 months and I'll have you in lycra.
Edit: Just saw your edit and 66cm, yikes the guy must be a giant or at least of
@Mike Rotch proportions.