I've been thinking this Galaxy has largely been unknown and has a connection to the Night Sisters. The Night Sisters are tied to the dark force users and that ring is highly advanced too advanced for the Night Sisters. It has me thinking maybe a group of Sith and Night Sisters ancestors worked together on mapping the new galaxy but those who made the trip could not return for some reason.

It's also possible that when they could not return, they conquered planets they found establishing their own Empire. I like this idea because I like the idea of there being Sith not tied to Darth Bane, and I like the idea of an Empire that's not like Sidious's and I can see Thrawn forming an alliance with such a group especially if he agreed more with it than the Empire of his galaxy.

Where I see Ezra coming in is for him to have joined it after experiencing a more peaceful life there and wanting to defend his new found life from any threat. I know how weird that sounds but if Ezra grew accustomed to a government and society that while not perfect was better than the one he left behind imagine what he'd due to protect it from someone who would completely oppose it. because it's not 100% like the Republic.

I can see the story being something like they both are found by Sith soldiers and brought in. Thrawn handles his situation fairly easyily. While Ezra takes longer to adjust and is slowly introduced to the new society via an assigned Sith lord. As for how things might be later I can see Thrawn in a similar position as he was in the old Empire and Ezra pops up later as a Sith warrior and surprises Ashoka and Sabine by capturing them and trying to get them to adjust like he did eventually revealing he's got a family now he doesn't want to lose.

I think that'd be an interesting route as it makes sense. It also would be a good form of manipulation by a sith society. Taking Ezra whose never known peace and letting him have a life akin to a dream that way he's got more to lose if his friends ever show up and pose a threat.