I think going to a large sensor compact would be a distinct upgrade to your current Oly rig. I have RX-100M2 in Ikelite housing which a little larger but works fine. I have looked hard at Canon G7X for my dive buddy and think that is a superb camera. Mirrorless maybe better from a system flexibility viewpoint, but more costly and cumbersome. Would be like going from grade school to college skipping high school~
Also take into account the wet lenses that are compatible with both RX and G7X. And the port system with the Canon uses.