Stripe integration for SaaS, do you use custom or prebuilt ones?

Well said. Stripe Checkout has all the value you need in the beginning (i.e. credit card processing, subscription plans, refunds, etc). If for some reason you grow to the point where it doesn't fit your needs (which is unlikely, since Stripe is incredibly powerful), then you can change it then—you should be making a good amount of money by then anyway.

If you don't make money off the project, you can shut it down without having wasted all that time and energy building a custom checkout solution.

In general, I advocate for never reinventing the wheel initially; i.e. using existing third party solutions for everything that isn't central to your app's "magic sauce;" the thing that makes it unique and provides the core value to your customers. That's the thing they're paying for. For any other feature outside your app, inventing or writing from scratch is an additional risk of wasted time and code to maintain. Reconsider these only when you know someone will pay for your product.