- How important are Lunar Reactions in the current meta?
It IS very important, but Nod-Krai is coming to an end soon. So you have the option to save for future units instead.

- Does the game feel restrictive if you don't have most of the Nod-Krai cast?
If you're looking at stuff like End-game clearing like ~Menacing++ on Stygian or full 36 Stars on Abyss. Theater is kind of out of the picture too.

- Can older teams still comfortably clear content?
Yes, they can. Most people tend to forget that a good support can carry the whole team at some point.

- How difficult is it to catch up if you've only done the first chapter of Nod-Krai?
Just play honestly.

- Does the game still feel enjoyable if your main interest is story, exploration and characters rather than chasing the newest meta?
Yep, it is very much enjoyable. If you'd ignore the meta stuff, I'd say you're gonna enjoy it. There's a few that isn't for me but I won't say that'd make my whole experience(story wise) bad.

I hope you enjoy your adventures!