I've been down the rabbit hole this morning, so a deep cut question here
 

Are Kamen Rider Aqua, Sea Jetter Kaito, and Bashaan (from Choujin Bibyun) all based around that same early Ishinomori sketch? They're very similar in design and motif

You might be on to something.

It's highly likely. Kaito is directly inspired from the sketch and KR Aqua was created at the same period where Toei was revamping several older characters like Skydain, Groudain, Inazuman, the Akumaizers.

I don't know a damn thing about Choujin Bibyun to comment, but I can see the similarities upon looking it up.