I've been playing around with a "Funny Bike". Think of your tri-bike, minus a couple of decades.
Anyway, there are a few critical adjustments like moving the seat forward to maintain the angle between the torso and legs. But, this adds weight to the hands.
I put bars on my bike that allow riding with several positions.
Roo.jpg
I wouldn't say the bike is particularly uncomfortable to ride. However, the forward seat position does put a lot of weight on the hands, and it is actually worse when riding on the tops of the bars than when riding in a more extended position in the drops.
You might be able to move the seat back on your Giant, and add a stem riser and drop bars to make it into a road bike, but you'll likely be limited with things like a proprietary seatpost.
Anyway, your Giant might make a great second bike, but perhaps not a primary bike.