Hmmm I don't actually think much about the ending of her story, I focus more on her life in Galar until the Darkest Day LOL After that is a little blurry to me but I have thought a little about it.

I always imagined that after the Darkest Day, Briar paid her way out of jail and probably had some deal with the Interpol because it was obvious that her identity as a former Flare Commander was revealed lol, but you know when you're filthy rich you can escape prison twice 🥴. But they asked her to keep a low profile for some time to not cause trouble, so she ended up moving from the busy city of Hammerlocke to the Crown Tundra. It was a calm place, and she probably did some personal research on the place's Legendary Pokémon for fun.

To be honest in a "game" setting, I always imagined you could find Briar in the Crown Tundra after meeting some requirements, like talking to Oleana in the mines, some days she would say something like 'Briar is once again in that place', like thinking out loud. "That" place is a location in the Crown Tundra that's important for them, and is the cemetery. You can find Briar there only during the day (as opposed to Oleana who appears at night) paying respects to one of the graves (who's buried there is kind of spoilerish). If you talk to her, she will remember you from the Darkest Day and will congratulate you for becoming the Champion, and will say that she's not researching anymore (lying as always) but she will gladly measure her strength against a champion, so you can battle her every time you talk to her (but she doesn't appear every day—to know if Briar is there, you can check by talking to Oleana first, something like Mirage Island lol).

Her Pokémon are very powerful and she even uses her surroundings to her advantage—she knows her opponent can't Dynamax without a Power Spot, but she will Mega Evolve Houndoom for the extra strength. :3c Unlike her League Battles, in this special battle Briar would go all out.

Anyway, that's everything for the "game" setting LMAO. Briar moves to Freezington, and I find it funny how at some point Briar would end up finding out that Peony and Nia are also there LOL She's not particularly a fan of the quiet life, but that's life for her

Also it's worth mentioning that she retires as a scientist and an idol after the Darkest Day so she's not really working anymore. Sometimes she's contacted by other professors/scientists but her help is not a 'real' job and is more like something she does sometimes. She enjoys her days doing nothing in the Tundra, sometimes she goes on adventures and probably saw some Legendary Pokémon. Her team likes living there a lot because of the weather.

I like to think that, in the future, when everything calmed down she moves back to Wyndon and opens a theater because she can't live in peace without working </3 And also because she promised Rose she would take him to a play at her own theater one day and she does :3. They probably travel to a lot of regions since they both have all the time of the world because they're not working LOL

But yeah I guess the ending of Briar's story is that she enjoys her married life in peace and doing what she always wanted to do. Which is a pretty normal ending but for her it means a lot because she never thought she could ever have a life like that, in peace, enjoying life and love and getting to work on arts which was always her biggest passion since she was a child. So for her finding this fulfillment in so many areas means a lot, so she's happy like this. ♡