Just bought the TG6 for my 13 year old son who also is a Junior Open Water diver. I set it up with a video light on a flex arm with a tray and handles. This apparently is a great set up for macro and portraits. Also the Olympus housing is not too bulky or expensive. Down the road you can add strobes and a wide angle wet lens.
I’ve got a GH5 (similar to the other m43 camera you mentioned) with a zoom wide angle to medium lens in a dome with two strobes. So together we have every type of shot covered.
I think this is a perfect setup because my son won’t have to worry about much but focus and pressing the shutter button, which is just as well as he is trying to maintain neutral buoyancy!
Here is a video worth watching on a similar setup, select Part 2:
https://www.backscatter.com/reviews/post/Olympus-TG-5-Best-Underwater-Camera-Settings#
I went with a single flex arm rather than a ball arm because again it’s easier to adjust. Backscatter also has a tether to attach the camera to a BCD, so it doesn’t drift away. The setup above has no floats and will sink.
We will be trying out the TG6 in French Polynesia in a few months.