I agree with going to Blue Hole daytrip with Amigos del Mar.
It is very likely that Chuck and Robbie's (near to Blue Tang Inn) and Reef Dive Adventures, right on the dock in front of Blue Tang Adventures, will send you on Amigos boat, even if you do all of your other diving with either of them.
Be that as it may, Amigos is an easy walk from Blue Tang, and you can check out their overall operation yourself.
If you will still be daily diving once moved down to Xanadu, whichever dive op you are using will pick you up and drop you off, at dock closest to Xanadu (sorry, can't recall off top of my head whether or not Xanadu has a dock right there but I'd be very very surprised if they don't!
While you are down at Xanadu (which I'm sure that you will love, wander down to check out Mata Rocks Resort too. Have a drink at their Squirrel Bar and get them to show you a couple of their rooms.
You have picked 2 of my favourite hotels in San Pedro.
If you have a day free, take the water taxi over to Caye Caulker. Who knows, you may much prefer the more quiet, slow pace on that island.