ActivityPub is a protocol designed for decentralization, but the Mastadon software itself really is not a great option for self hosting. In practical terms, for non-technical users (i.e. the people you need for broad adaptation) self-hosting it is a non-starter.
The same is definitely true for email. There are many providers, so "centralized" is a bit of a loose label, but running your own email server comes with non-trivial pitfalls that keep most people away.
Use Pleroma instead of Mastodon, it is quite easy to host and only takes a fraction of the resources Mastodon does - it idles at 165 MB on my test instance (single user logged in, no activity). From what I've seen it should possible for non-technical people to self-host it as long as there is a possibility for remote-access assistance (and there's your business model, sell wall warts with self-hosted services with optional remote assistance).
That said I do prefer a mail-based system like Delta Chat [1] over these ActivityPub-based things since everyone already has a mail address and there is a wide range of software to choose from.
The `nostr-rs-relay` package is a simple self-contained rust application with a sqlite db that can run in a docker container. Super simple to self-host and if just using it to save your own notes is more than enough.
