Ultra Sun and Moon went from a story about Lillie's family issues and her growth as a person to a story where all of Lillie's character development goes into the toilet because an alien space prism decided it didn't want Lusamine as the main antagonist anymore. I'd say more, but I'd like to avoid going on a rant.
That's not what happened. Lillie still had around the same development as she did in Sun and Moon. Minus Exeggutor Isle being cut out, most of what she had to say beyond background between her and Lusamine was moved over to the Murkrow Bridge. (Her I want to become a trainer part.)
We actually got the implications that Lillie called her mother out on her actions... all be off screen this time, with some of the things Lillie actually tells you prior. (Her conversation with Plumeria.) And the things we find in the post game and after RR shows how a combination of things, what Lillie said to her included, ended up affecting her and causing her to change for the better.
Lillie still has her farewell to Nebby as well, at Mahalo Trial this time with more reasoning behind it. (You and Nebby needing to help Necrozma rather than... out of the blue.) It's available right after the grand trial and Gladion points the way.
And I had issues with how SM ended for Lillie, she stood up to her mother... and WENT BACK TO HER IN THE END. Ditching all the friends and family she made on Alola I might add... and including Gladion who got stuck running the place he was abused in.
In comparison to Ultra, YES Lillie forgave Lusamine in this one as well, but after tending to Nebby, the two gone their separate ways. Lillie went back to Kukui's place and became a trainer proper. (RR with her Clefairy and to add in, she also has a cute crush moment after all is said and done.) And after RR is a damn good battle tree partner.
Lillie also stood up to Faba, stood up to Ghetsis (even when he was threatening her.) and was willing to protect her mother when push comes to shove... rather than just having you do all the work. (You are helping her as much as she is helping you.)
And the Necrozma thing taking Lusamine's place as the big bad. That woman is still the reason Necrozma went from a looming threat to an active threat, still the primary antagonist for most of the game and ended up being the reason Necrozma came and tried to eat the sun, taking over the spotlight.
I also argue that Lusamine never getting sick lead to some development for that woman along with some closure for the family, rather than just having a near death and us never hearing anything more from her again along with some lose ends left behind. So its a trade off I liked and Necrozma was a FAR better climax boss in comparison to the mother beast. (Things are actually at stake for you and the whole of Alola and you fight the fused monster this time.)
The biggest problem with Lillie and her family issues in Ultra is that it just wasn't the main focus this time, it's still a major part but you, Necrozma and the whole of Alola are the bigger focus.
You could argue that Lillie not being the center of the story is a downgrade, (Since it her arc is less detailed and less emotional.) but I think you and a lot of people are over blowing the damage and getting things wrong.
As for the big topic. My input is while third versions were a thing back then, they should have just been in the base games from the start and at the Switch Era, we are talking 60$ for what would be mostly the same game! The expansion packs were needed.