I was running a federated synapse on a much lower spec'd machine than that ... and it was fine. I don't think it's federation that does it, it's joining large and active groups.
The author is actively answering questions on the Reddit thread, probably best to ask them.
It has sieve support and rspamd/spamd are supported via filters. It doesn't require ldap. PGP doesn't require any server support so that should work fine. I haven't seen anything about supporting encryption at rest.
JMAP was developed by the guys that run FastMail (who are the primary developers of the open source email server Cyrus-IMAP). It's easier to implement and more performant.
Side note, Cyrus is a pretty amazing mail server. It doesn't get much love here, but it's bombproof, fast, supports multi-node clusters, IMAP, NNTP, CalDAV, CardDAV and more that I'm forgetting. It's just a bit old school as far as configuring goes.
I think there's a lot of FUD around this. Yes, deliverability can be a PITA, but with a clean IP and good setup it's usually solvable. Worst case, you can pay a small amount to use a 3rd party SMTP relay and still get most of the benefits of selfhosting. It wasn't deliverability that made me stop selfhosting it was spam, and it wasn't that dealing with spam was that hard, it was just annoying.
Caddy is server subdomains and standalone domains, doesn't seem to make a difference which is which .. neither work. For the moment I'm monitoring them from another server but that's annoying ... which I could figure out what I'm doing wrong!
Personally, I'd rather just block the stuff I don't like rather than have lemmy.world trying to decide "who's worthy of federation".