With Connect account creation is it necessary to use the Account Link flow to gather all the necessary information or the all the data can be filled in via the accounts API without client interaction?
The AccountLinks and onboarding flow will be necessary here. You can prefill some info on Standard Accounts but a lot is not settable as the platform via the API.
For accepting payments we want to continue using our own payment page - so not using Stripe Checkout (and likely not Payment Element). Is it possible to continue using our own payment page with Connect Standard? We already pass the card/billing details using PaymentIntent with the current integration which isn't using the Connect account.
Yes, you can do this with direct charges. You will create the payment intent but on the standard connect account. You then pass the client secret to the client side, initialize Stripe.js but for that account, and can confirm the payment with the payment element as you do now. https://stripe.com/docs/connect/creating-a-payments-page?ui=elements&destination-or-direct=direct-charges
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