...I hear the diving in Turks & Caicos via live-aboard tends to be deep; the Cayman Aggressor IV or one of the Belize Aggressors sound like better 'starter trips' to me, with T&C maybe later?...
IIRC then most of the walls in the Caymans do start more shallowly than in the T&Cs, and I remember on our most recent T&C Aggressor liveaboard trip that a few of the divers stated they preferred staying near the top of the wall on some dives. And I recall one couple declining one or two dives because they were a little too deep for their preference.
But after checking my log book I see that most of our T&C dives were in the 60 to 100 foot range, some a little deeper and some a little shallower, but that was the typical range and about the same as our Cayman Aggressor dives. I think it may be a bit harder to find shallow sites in the T&Cs, so the Caymans may be better for a new diver or someone that just prefers shallow dives. But I think that most recreational divers will still be able to find a comfortable depth range in the T&Cs.
I had issues with air consumption rate when I was a newer diver and I learned to prefer shallow depths just to make my air last longer, but I was okay during our first T&C liveaboard trip more than 20 years ago. Even though air consumption is usually not an issue any longer, I still often tend to "shallow up" as the dive progresses and I've never had "too deep" issues in the T&Cs.
If the T&Cs are on your Caribbean diving bucket list then I suggest a liveaboard trip. That may be a bit presumptuous because I have never done any land-based diving in the T&Cs and a lot of people say that it's great. But I've read about long boat trips to the reefs and heard that the best diving isn't available from a land-based operator. The liveaboard diving around French Caye and West Caicos is fantastic. Beautiful healthy walls, lots of sea life, and many large animals including turtles, eagle rays, morays, and sharks on just about every dive. A bit different from Little Cayman and not many "friendly groupers" but on the same level of excellent diving as Bloody Bay Wall IMO. I have never done any diving from Grand Turk or the other T&C out islands so I cannot comment about those sites.
Perhaps someone that has spent more time diving the T&Cs and the Caymans can provide more information.
If you do take a T&C liveaboard trip some day, try to spend a little time on Provo, it is a lovely and enjoyable vacation spot. It is less developed than Grand Cayman but it is just as expensive IMO. Grace Bay Beach is fabulous, even nicer than Seven Mile Beach IMO.