Nice ideas, and I use a similar setup for my personal domain. For people who don't want the hassle of setting up a domain name, maybe there should be a standard by which an email provider could be queried to find someone's social network account provider.
If your email address were, for example, ocd AT trekk DOT ie, an email contact of yours should be able to make a DNS (or SMTP?) request to trekk.ie and find your social network account ocdtrekkie@mastodon.cloud, where they could send a friend request from their own social network provider. The link between email account and social network account could be checked periodically based on the TTL.
There is a somewhat analogous IETF standard for looking up identity data from mail servers, here: