Reddit mentions of Origin8 Pro Fit Alloy Ergo Stem, 1" or 1-1/8" x 31.8-25.4/28.6-25.4 x 35d x 110mm, Black

Giant TCR is a pretty nice bike, and although the geometry is not endurance fit, it's not a TT or Tri bike. I'd keep it. If you want to bring up the handlebar or closer, you can change the stem. I don't think it's outright wrong. But, if you want more relaxed geometry, there are other options for sure.

I have the Origin8's ergo stems on two of bikes. Because the angle 35d rather than standard 7d, I can ride more upright.

Unless you strongy dislike the TCR's handling, that's what I'd do.

Between the Impulso and Synapse, I'd prob. take Synapse. (I had 2010 Synapse). To be honest, I don't know Impulso, but I do know Synapse is said to be the best alu. frame bike.

I don't know your budget, but I'd take a look at Trek Domane, Spec' Roubaix, Fuji Gran Fondo, Raleigh Clubman Disc or Record Ace, Giant Defy, Scott Contessa Speedster. One odd ball like Lapierre Sensium is also an endurance bike. (I happen to have Sensium 400, and love it.) If you can find Jamis Endura, Bosanova, try them too. Pretty every bike company has an endrance oriented bike and it's up to your budget and LBS's availability. For the money, Giant, Fuji, Scott are better values, IMHO.

I'd prob. pick a steel frame bike with Tiagra, like Raleigh Clubman Disc. A little more comfy than alu. frame, and Clubman is really made for group ride. 105 is nice but Tiagra, esp. current crop is very nice and fits your needs better.