My wife and I tried to couch co-op from the start. It did not go well. The split screen and neither of us knowing the controls made for a difficult experience. She lost interest, and now is even "meh" about watching for the story. May want to try starting a campaign by yourself so you can learn how to play and help her get into it easier.

Can someone join you mid campaign?

Or do I have to start new when I bring a couch co op player in?

She can join in later! When she connects her controller it will let her create a character if she doesn't have one yet. I think she can also join with an established character if she sets one up on her own profile first, but can't confirm.

Once she has a character in your campaign, you can play solo with her character still in your party for XP and loot. You just lose out on being able to have one of the established origin characters in your party.

About 50 hours into BG3 on PS5 and damn, what a fantastic game. Feels like I've barely scratched the surface. New to the D&D type of game, so there was a steep learning curve and I'm still figuring things out as I go. I kind of flailed through the intro / ship part of it, so ended up restarting after awhile and got the Everburn Blade, which is well worth it. Still wandering the Act 1 surface map and haven't even started the Underdark yet.

Finally made it to lvl 12 (vs restarting repeatedly).

Happily blowing things up and making bad decisions. It was the only way to get this far, settling with your decisions. Just a small amount of save scumming, but I got there.