You can book speciality restaurants 120 days out, either on the website, via chat or on the app. It is not necessary to book complimentary restaurants unless you have a large party and I would not bother for this type of cruise, as you will have very busy days. You might want to grab something quick in the buffet, or if you get back earlier try the MDR.
Wear what you want but trust me, on these types of cruises, I am lucky if I get a chance to freshen up before eating and going to bed. I love food tours and trying to local cuisine off the ship so that by the time I get on, food is the last thing on my mind.
Enjoy your cruise.