Idk, I don't feel it's too rushed. Maybe a bit, but they had some teasing in prior episodes and I don't expect pokémon to really go the fluff-route and I'm not exactly the target audience. Most young kids don't really understand romance yet, and filling the show with slow fluffy buildup might have been more realistic, but I don't think kids are going to be interested in that. Kukui is after all a supporting character, he's not the character the kids identify with, they identify with Lillie, Ash, or the other kids. They don't want to see episodes filled with the old professor Kukui walking on the beach with Burnet. They want to see pokémon doing funny things, pokémon having cool battles, and the kids being cool and kind role models. And that means simplifying relationships and marriage a bit. Kukui and Burnet really like each other and are in love with each other, so Kukui offers a ring and they get married. It's a simple enough concept for children to understand, and I don't think it needs to be more fleshed out than that for the kind of show that pokémon is. Would it be appropriate like this for an adult show? No, but this isn't a show for adults. It's a show for children at the age of 7 or so.
Let's be honest, it's implied that at least some time passed between episode 54 and 55, during which Lillie didn't go to school yet because she was helping out her mother, and Kukui and Burnet spent a lot of time together off-screen during that time. I can say, yes, maybe they rushed into marriage quite fast, but it's never exactly stated how long Kukui and Burnet already knew each other, nor is it stated exactly how long the time skip between 54 and 55 was.
As for why Kukui and Burnet weren't already married at the start? I don't know, they could've taken that route. That'd imply that Burnet would also be around a lot in the earlier episodes, and my guess is that they didn't really have anything for her to do during that time, so they waited until the Aether arc where she'd also have more plot reasons to actually be there and have space to get to know her when we already know most other characters like Lillie, Mallow, etc. and have a cute wedding then.
As for Faba, Lusamine stated that he got demoted, he's now essentially Wicke's lackey, similar to what happened in USUM after Rainbow Rocket. I can't be too mad at it. Is it realistic to keep a guy like that employed after what he did? No, realistically he should end up in jail. But meh, this is a kids show that wants to promote tolerance and forgiveness, feelings of vengeance and revenge aren't to be encouraged. It's not like Jessie, James, and Meowth ever get jailed by officer Jenny either.
I do think that Lusamine, Wicke, and Faba asking the kids to join their efforts with the Ultra Beasts was a bit oddly timed. I don't think doing it during the wedding was very appropriate, I'm not sure why they put it there. I think they should've put it after the wedding was over.