With payment processors like Venmo, Stripe, Square, etc. that simply process payment transactions between two parties, but do not hold funds (and therefore can't be tracked as an account in Quicken), add a field to transactions that identifies the platform through which the transaction was processed. As a small business owner who uses these extensively, and now must reconcile 1099-K reporting from those entities to the IRS, it's important to have this tracking capability to make correcting adjustments to those 1099's so that only taxable income processed through the processors reaches my tax return.
This could be accomplished by adjusting the Check number field so that drop-down picklists can be user-created: Venmo, Stripe, Square, e.g. and those transactions then marked accordingly in lieu of assigning a check number (which does not apply to these).
Associated tax reporting also could be built to tabulate, for example, all Square transactions with category that has been assigned as a tax-item.