Cheatcode for flavor is chicken bouillon. Powdered chicken stock has almost no calories but packs a ton of non-specific savory flavor (umami) that you can put into any savory dish. If you feel like a dish is bland, you salt it and it still feels not salty, try adding chicken bouillon and the flavor pops out. It's why Australians go mad about their "Chicken salt" which is mainly bouillon and salt. (If it still taste bland after this point, you probably need some form of acid like a vinegar in the dish.)
Tuna itself is pretty generic in flavor so just go ham with spices and seasoning. I like to mix my tuna into scrambled eggs (1 whole egg + 200g of egg white), mix it with gochuchang red pepper paste for big flavor, mild heat, and slight sweetness. You can also use Laughing Cow cheese wedges for about 45cal. Mix that into the pepper paste with a little bit of water to turn it into a creamy consistency. You can use the cheese wedge trick or cornstarch slurries to make creamy sauces/gravies for whatever dish you want with minimal calorie cost.
There's a lot of exercise programming you can do to optimize your fitness, but when we know diet is like 90% of the aesthetic outcome, leveling up cooking skill is probably much more important for creating tasty lean food you have no problem adhering to.