This'll be unpopular, but if you want to keep it super lean and avoid being asked for compliance certs like SOC2/ISO, you could consider building it as an installable app on top of a platform your customers already trust
ie. a Salesforce App.
That way, they already use/trust the environment where the storage/processing of their sensitive data is taking place, akin to an old school 'on prem' solution (but without as much headache for you)
Worth thinking about
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