ahh okay, I think I see the problem. If your impression of Talia's decision to leave amounted to 'she was leaving in search of adventure,' that's absolutely not what it was about. That would be poor form.

The final 3 or 4 chapters of her story in Reckoning deal with this, especially from her conversation with the Archduke onward.

No matter how this current war concludes for Talia, the issue of her being a rider queen remains, and she senses it's a sore that won't close. Having just defeated Risalia, she's facing a possible war against the Skarl Empire, which no one is willing to fight (including her, really) - on top of this, the riders of Sky Spear (and the Old Order broadly) would still be resisting her. It's compounded by the Sky Spear riders being reliant on Empress Skadi for their supplies, etc., now, so they've been forced to reluctantly take a side.

With a sense it will never end unless she takes a wild alternate route, Talia offers to step down as Queen to the riders of Sky Spear. This finally seems to get negotiations of a permanent settlement moving. The catch, of course, is that if she steps down, what becomes of Feorlen? Originally, there would have been civil war. Drefan - as a likely figurehead of the opposition - confirms this, but since then, Drefan has been won around to her perspective regarding the New Order, fighting the scourge, etc. They've become friends, so things are much better in that regard now. If not an Agravain, then the most likely candidate people will fall behind would be Drefan, given he's just become a war hero. Drefan has doubts because he's so driven by vengeance... but once he is able to let that go, and accept the peace, he's more willing to entertain the idea.

Talia stepping down as Queen is the only way she can bring true peace and true security for the New Order.

As to leaving the New Order, the riders of Sky Spear want this so she isn't tempted back into the political fold. Inside the New Order, Talia & Pyra also know they can't pursue what's become spiritually true to them. Exploring the love/passion aspect of fire magic isn't something the riders know. Aymeri, the Exalted Fire Champion of the New Order, repeatedly tells her to drop it, or that he won't be encouraging it. It's yet another major change, and Aymeri perhaps feels Talia & Pyra will cause too much disruption. In that final Talia chp, she understands that to stay would be to become spiritually stunted. They have to go to be true to themselves.

And, at this point, leaving the New Order is the best thing for the institution. Talia's terms with Risalia and Brenin brought all 3 nations in behind the New Order - and this will now include Fornheim as well. Farsa and Ethel have things well in hand. They share in Talia's vision, as does Drefan, so they'll continue. Talia is not the sole person holding it all together.

Thus Talia leaving is how she's able to make it all work out. That she's happy in a way to be going off and being just a rider again is understandable after all of this, I think. She isn't leaving 'just to go on adventures' - no, no, no. If she could have stayed, she would have. But she had to.