Have you thought about Utila? I understand from the experts that May and September are the absolute best months for seeing a whale shark in Utila. I believe you have a better chance in Utila than Belize, in any case. From what I understand, you go looking for whale sharks during your in-between dive times, so you still get to dive even if you miss out on the whale sharks.
Have never done it, but I've done a bit of research on this myself. The ride from Hamanasi to Gladden Spit is quite long. Manta is closer, but there is still no guarantee and the added cost is, IMHO, not inconsequential. OTOH, I have been diving in Belize (not near Gladden Spit) and it is wonderful. So even if you miss out on the whale sharks, there is plenty else to see and do.
As I've said, I've never been to Utila and have never tried whale shark diving, although we are extremely anxious to do so! I have read a lot about it and talked to a lot of other divers about it. (Recently met some people in Cozumel who were very happy with their experience in Utila, which is why I mention Utila to you.) I wonder if there is anyone out there who has been to both Utila and Belize for whale sharks and can share the experience?
I'm honestly not trying to confuse you more, but can I just say a few words in defense of shore diving? I've never been to Curacao, but I have been to Bonaire. Like you, I didn't feel confident enough to go to Bonaire when we were truly beginners. Once we did go to Bonaire, some time later, I realized that I was wrong. As long as you stay away from some of the tougher sites, it really is quite easy. It's easier than any other place I've ever been, and the diversity of marine life (except the big stuff) and the viz is astounding. Personally, I find entering and exiting the shore easier than a lot of the boat dives I've done, with the current and the chop and trying to get back on the ladder without getting smacked on the head. And there are boat dives to do as well in Bonaire, so you have a DM to point out the frogfish and seahorses for you.
In hindsight, I wish we had finished our AOW in Bonaire. We went to Roatan instead--great instructor but terrible weather, a lot of the sites were deeper than we could go, so it was not the best choice. Bonaire is out of the hurricane belt AND it is really dry.
Okay, enough! I don't work for either the Utila tourist board nor the Bonaire one, honestly! Hope this helps!