Children's mystery/adventure book series similar to Enid Blyton's work

I'm looking for a book I read in either 2008 or 2009 in Indonesia (but it was definitely a Western publication and entirely in English) that was part of a series extremely similar in format and premise to The Famous Five and other Enid Blyton books. It featured (as far as I can remember) five middle school aged kids who regularly go on adventures together. There was always a section at the start of these books that had illustrations of all the kids with a brief description of their personality. There was a boy named Dave who was in a wheelchair and liked to play basketball, and a girl named Cathy or Kathy who was described as shy. I think there was something about them having crushes on each other? Unfortunately, I don't remember details about the other members of the group, just that they were mostly boys and I think they were all friends from school.

The particular book I read was about them going to some kind of island or beach and trying to find a treasure. They befriended a local boy who was friends with turtles and could scuba dive, and they all eventually found that treasure in an underwater cave. The title had something to do with a hidden treasure or secret cave. The cover of the book featured a somewhat realistic (like the Fear Street book covers, but less so) illustration of this scene where one or two of the boys were surfacing from the water in this cave, and it was a close-up of one of the boys' awed expression staring down at the glowing treasure. All the book covers in this series were in this semi-realistic style.

Every time I try to look it up, Google just directs me to other vaguely similar children's series, so any and all ideas/suggestions are very much appreciated!

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