As a self-reply, i will say that Migadu having a permanent free tier is very nice. Not that i intend to use it exactly, but knowing that if i go broke and homeless, still having the same "internet contact address" is useful to me.
Not that the pricing is likely to ever be an issue (knock on wood), but i'm a planner.. of sorts.
Just understand that there is a large risk to a service that has a free tier as its draw. Free accounts have resource demands that cost money. If the company becomes insolvent, your "permanent" account ceases to exist. Fastmail is appealing to many in part because it is a paid service. Regardless of the number of users, the service remains funded and will continue to exist. (Unlike the many startup services that have been pulled due to depletion of investor funds or user loss.)
Not that the pricing is likely to ever be an issue (knock on wood), but i'm a planner.. of sorts.