I sometimes cook for a good friend on a FODMAP diet, OP, and this does sound similar.
I think some of the ideas above sound brilliant, particularly the Salade Nicoise made with tuna steaks.
Can you eat butter? If so the Waitrose bagged fish sound good, or you could make something similar yourself - or you could use olive oil or margarine.
My secret for FODMAP cooking is mayonnaise. Homemade is best but any good brand will do; we really like Waitrose Essentials. It keeps things moist, provides creaminess and carries flavour. You can mix it with good olive oil if you want that flavour or texture.
Eg Mayo with lemon and dill (and possibly olive oil or mustard) smeared on seasoned fish fillets such as salmon or sea trout, then grilled. Leave out the mustard and dill and use tiny capers or chopped big ones - this is good on sea bass or bream. Use mustard and lemon, if you tolerate it, or lime and honey, on chicken pieces. Experiment with gentle herbs and spices. Etc.
Good chicken, beef and vegetable stocks, made with ingredients you can tolerate, are important. They add so much depth of flavour: experiment with using stocks to cook rice, vegetables, etc as well as making soups. You might be able to buy suitable stocks.
It isn’t easy. Good luck.