Really? Trust to send an email with a link? What else is making it trustworthy?

You’d be surprised how much trust people place in legal departments, balance sheet strength and talent capacity. All things for which I had to turn down superior technical proposals in the past. The old saying „Nobody gets fired for buying IBM“ still runs strong.

Free e-signatures are a great idea, have you considered getting a foundation to back the project and maybe taking out some indemnity insurance, perhaps raising a dispute fund?

That big companies use it for their important legal contracts.

its a well recognised tool for contract agreements, and you pay the money so that you are indemnified for any oopsies that might happen in transit.