To directly answer OP's question, the Defy 2 disc would be a perfectly great first road bike. I know several people who got Defys for their first road bike and were very happy and transitioning from hybrid may be easier than getting a more aggressive road bike. I have not tested the new Tiagra but supposedly it's functionally and cosmetically identical to 105 but in 10 speed. The Defy even comes with Shimano cranks instead of going cheap there and TPS Spyre are supposed to be very good disc brakes for the money. Giant generally gives very good value for the money.
As for budget and what you should spend on a first bike, that's up to you. You know what you can afford. The more you spend up front, the longer you will probably keep it if you get into it. A lot of people here want to make decisions for other people that they shouldn't buy carbon for a first bike or shouldn't spend over $1000 or over $700 or should buy used etc but everyone's situation is different. I have plenty of friends that spent $2000+ on carbon bikes and some road them 5 times and others really got into it but they could afford to spend the money so what's the harm? I know another guy that bought a new $1500 bike and fell in love and bought $3000 bike 6 months later and then bought one that cost around $8k the next year. But that guy has money and he also rides well over 200 miles/week now. Should someone tell him he shouldn't spend that much on bikes because he's only been riding a couple of years?