I think the best solution to this, given the requirements you outlined and the need to avoid wasting time on payment processing, is to use Cashu to build a 'trust' relationship based on certain terms, such as a predefined mint for payments. This way, consumers and providers can agree on a mint, and only payments with tokens from that mint will be accepted. This removes the burden of finalising payments, and tokens from that mint can be streamed, meaning those payments could be as fast as reducing the price from a local balance. This only works if a trust relationship is established between the parties: 'consumer -> mint <- provider'. If this trust relationship doesn't exist, you have to redeem Cashu tokens for every payment to reduce counterparty risk.

Another approach would be for you to run your own mint with a custom denomination (or just sats), where customers could buy tokens by minting sats. This approach has some interesting benefits, such as improving the privacy of your users, as the tokens act as both payment and verification. This means that you wouldn't need API keys and could act as a blinded custodian.