This is a better solution, but it still requires the trustee to be on Proton, which, IMO, is an unnecessary obstacle.
It seems to me that you’re better off writing your Proton credentials down in a secret physical document and tell your trustees where and how to access that document if anything happens to you. You could potentially give it to a lawyer.
That, or have practice drills one or twice a year with your trustees, which should also prevent free Proton accounts from expiring.
It’s still tricky if your trustees don’t live in the same city or country as you.