I'm not so sure about the "train your own model" advice. This sounds like a good way to set your product up for quick obsolescence. It might differentiate you for a short period of time, but within 6-12 months (if that), either OpenAI or one of its competitors with billions in funding is going to release a new model that blows yours out of the water, and your "differentiated model" is now a steaming pile of tech debt.
Trying to compete on models as a small startup seems like a huge distraction. It's like building your own database rather than just using Postgres or MySQL. Yes, you need a moat and a product that is difficult to copy in some way, but it should be something you can realistically be the best at given your resources.