Just wondering if you already have a buddy lined up for shore diving on Brac? The Cayman dive ops usually insist on buddy pairs for boat diving and they will pair you up if you are a single diver. They may also insist on a buddy team before renting tanks for shore diving, I'm not sure about that.
My understanding is that it is cheapest to sign up for the full boat dive package with Reef Divers in advance (2 morning tanks and 1 afternoon tank) and that it is more expensive to add on extra dives after you get there.
Since this is your first trip to Brac, and you are not sure about doing shore dives, you might want to take the full boat dive package this time and check things out while you are there to plan for a future trip.
We always stay at CBBR and we rent a car for one day to explore the island. It's not large and you will have time to look around. The front desk at the hotel will help you to rent a car, but insist that it has good A/C because we had to return a car one time.
Just an FYI, if you plan to spend any time on Grand Cayman and rent a car, save your temporary driving permit (20$ I think?) because it is good for a month and you can use it on all three of the Cayman islands.
Have you been to Little Cayman? Diving the Bloody Bay Wall and Jackson's Bight is spectacular IMO. We love both Brac and Little and if you have the chance to take the 2 tank trip to LC for an extra $35 be sure to take advantage of the opportunity.
The 2 islands are only 5 miles apart and if enough guests are interested and the weather is good they will take a boat over there one day for the 2 morning dives. Reef Divers is also the dive op at Little Cayman Beach Resort and the two hotels are owned by the Tibbets family so they make mutual accommodations. If enough guests at the LCBR want to dive the Tibbets wreck (the Russian Frigate) they will take a boat over to Brac for an extra charge.
The Tibbets is a nice, easy wreck dive, so make sure that the boat you are assigned to plans to dive it one day while you are there. If not they can temporarily move you over to another boat so that you can dive it. There is no extra charge to dive the wreck if you are staying at CBBR and diving with RD. The only extra charges are for night dives and the trip over to LC to dive BBW.
When are you going to CB? Have fun!