I started playing the original campaign of NWN2 and I'm not super happy with my class and race, a half-ling Spirit Shaman, and I'm thinking of trying something else when playing Mask of the Betrayer.
Good idea, or will I miss important/funny story beats?

Hey, your game, your rules. If it's not keeping your interest, it's cool to try something else. Play a Favored Soul/Stormlord as an Air Genasi. That expansion introduced the Genasi as a playable race and that base class and prestige class were two of the ones introduced with it.

Half Orc Barb4/ Bard 1/ RDD10

So I won't meet a returning character that will be like "Hello remember that thing you did in the first campaign? Oh you let that enemy go and didn't kill him? Weird... It's almost as if all you decisions you made back then have been erased."

No, at least not in any consequential way. There is significant reference to something you know about yourself 20 minutes into the OC and then they probably make reference to your being the hero of whatever which you can more or less figure based on the fact you have supposedly lived through it to make it to Mask of the Betrayer.

Good idea. The only "BUT" that I can remind it will be your starting lvl which should have enough XP for a lvl 18. If you finish the OC you should be already on lvl 20. And something similar with the items.

Frankly, it's not a big deal.

Awesome! Let me know how it works out for you.