You'll find one strobe will make a big difference in your photos. The TG system hasn't changed much from the TG-4 to the 7. I wish OM System would come up with a real new and improved version, with a bigger sensor and better manual control. It's an excellent system as is, but we can always wish for more.
The Sony A6000 is older but still usable. I have one and use it but it doesn't compare to the focusing capability of newer Sonys. I also have a Sony A7c. If I had to choose between the TG system, the Sony A6000 and the Olympus E-M1 or newer - and could only use one - I would go back to the Olympus mirrorless. They're small, light, great ergonomics, wonderful lenses, and cheaper to put in a housing.
Good luck!