First off congratulations on the upcoming wedding!

For Phuket, if you‘re going to be on Siray, might as well consider going to Koh Yao Nai/Yai? I‘d say it‘s worth spending a few nights close enough to Phuket old town/city to enjoy some local eateries. Then either go somewhere quieter like Mai Khao as others have mentioned or on either of the Koh Yao. Koh Yao would give you the scenery of Krabi as well.

For Bangkok, I second the suggestion that you should consider spending some nights at a riverside place to be closers to some of the sites there and the other nights near the Siam/Chit Lom areas are fine if you want to be in the middle of things.
Alternatively, if you‘re trying to use Bangkok as a base to do day trips outside of Bangkok, you might consider staying near Ari.

You can sometimes get unpublished deals in Thailand by contacting reservations directly and ask. They also may have things on Line/Facebook that are not even on their websites. Also, if you don‘t mind staying at the same chain throughout the country, it‘s not uncommon to get special discounts/promos if you‘re doing multiple nights across the various properties of the same group and also for a special celebration. E.g. Anantara, Cape&Kantary, and Centara groups have properties at places along your itinerary.