This is mostly just an issue with TROS since there was never any indication in the previous films that Leia had been trained as a Jedi, other than the space walk in TLJ and even that’s a stretch.
It would be an interesting angle if Leia’s decision was portrayed as a failure, but this is supposed to be a hopeful scene, “Leia failed the galaxy and doomed us all by choosing a career in politics” doesn’t fit a scene that is supposed to restore Rey’s spirit. To be fair neither does a premonition of death but that’s why we removed that bit.
I don’t think this dialogue really invites you to think of it as a failure, it’s not really saying anything we don’t already know/suspect about her (since TFA really, because we know she must have made that choice after Luke revealed her potential). Which I think is kind of the goal, it’s maybe a bit much to add some Leia lore (with premonitions or something) when there’s not really any time to explore its implications.
I mean, yeah, the dialogue doesn’t present it as a failure, but it doesn’t take much to come to that conclusion, why did she pick politics when she would’ve been much more helpful as a Jedi?
Well the idea that a Jedi is much more helpful is not really a point of fact. Seems more like Leia sought out Luke when she realized the Resistance wouldn’t be enough to stop the First Order alone. And even if the Jedi were around, they alone wouldn’t be enough either, they needed the Resistance too, and that came from Leia.
What good did her diplomacy do to the New Republic that was obliterated two movies ago?
Again this is a decision we already know that Leia has made going on two movies at this point. Unless the issue is with the specific phrasing of “best serve the galaxy,” I don’t see how this line changes anything.
This movie already added Leia lore by having her training with Luke and teaching Rey, I’m just looking for a way this can be nudged so it aligns with what we saw of her before.
I don’t really see a functional difference between Leia testing out being a Jedi and not even wanting to try to be a Jedi when the result is the same, that for decades she has not been a Jedi.
Leia being Rey’s master doesn’t seem that big a deal. More like she is helping out with her training with what little she knows. Rey calling her master is more out of respect than anything.