I got an email today about the CC Holiday Special from Deep Blue Adventures (for some reason I can't find a single link to copy) but it says that the packages include trips to Brac from LC to dive the Tibbetts and to LC from Brac to dive BBW at no additional charge, weather permitting. And I am sure that you can add-on the afternoon dive and/or the drinks package for an additional fee - probably up to the $300 extra that you mention.
I think the advantage of the Holiday Special is that you can book through Jan 17, 2020, which is high season. All their other advertised packages end sooner with the longest date being Dec 31, 2019 on a couple the more expensive packages.
I will be interested to see the price on the new flights from Miami to Brac/Little Cayman, to learn if they offer any additional savings.