• There was this part in the Emerald City sequence where these two women appeared on screen, and the audience cheered. I'm not familiar with the musical, so I'm assuming they are the actresses from the original.

Lol, yes. It was Kristin Chenoweth who played Glinda and Idina Menzel who played Elphaba. Idina also did a quick "ah-ahhhh", which is the final note sung in the film, which was a fun little "spoiler". (Idina also plays Elsa, so if it sounded like Frozen, that's why.)

• The setting of Oz, with all of its magic and whimsy, was so...inspiring. The songs, the costumes, and in general the vibes...whatever this is called, I need more of this in my life.

Steampunk, sorta? They definitely make everything more colorful, almost neon in a way.

I actually missed the first few minutes of the movie, but I made it just in time for the opening number and the title card.

I think all you missed was the setup that the Wicked Witch had been killed by water, as reported by Glinda. The rest of the opening explains in more detail, so plot wise not much missed, but the tone is one of sadness.

Glinda was such an unlikeable character at first, but I guess that was intentional to show her character development

You got it exactly. Both Glinda and Elphaba grow, specifically due to each other. There is a particular line discussing each of them "not having a care in the world" and the other realizing that no, they really do care.

My only complaint is that it felt too long

It's funny they didn't add too much from the Broadway show and yet the whole story will end up being double in length. However adding a quick scene here, an extra minute there and it quickly adds up.

As someone who complained about them splitting the movie up into two parts, I get it now. They really gave scenes time to breathe. You still could bring the runtime down, but once you hit 3 hours, it's either trim or breathe, and I think they made the right choice.

The second film will likely be "worse", but will have the advantage of tying up the loose plot lines, so it'll even out.