… but I am now looking to create a Custom GPT with this knowledge to see how it performs.

Why? It doesn’t make sense to use a LLM for soccer betting. What model(s) are you currently using? “Mix of AI” doesn’t sound credible since no MLE would say that so I hope that’s just for marketing bs purposes.

… or even build our own LLM.

Don’t do this, you don’t need a LLM to generate a fitness plan. It’s like getting groceries with a monster truck. Sure it can work but its totally overkill, unsophisticated and expensive. Also, generating meal plans with with LLMs can be dangerous since its mathematically inaccurate. You don’t need AI to generate a meal plan or fitness plan unless you want them to be hyper personalized. In that case, you need recommender systems, not a LLM.