The biggest certainty is that just having an open port for an SMTP server dangling out there means you will 100% be attacked.

True.

Not just sometimes, non-stop.

True

So you don't want to host on a machine with anything else on it, cuz security.

I don’t think "cuz security" is a proper argument or no one would be ever listening on public internet. Are there risks? Yes.

So you need a dedicated host for that portion

Bullshit. You do not need a dedicated host for smtp ingress. It won’t be attacked that much.

and a very capable and restrictive intrusion detection system (let's say crowdsec), which is going to take some amount of resources to run, and stop your machine from toppling over.

That's not part of the mail pipeline the OP asked for.

Here, I brought receipts. There are two spikes of attempted connections in the last month, but it's all negligible traffic.

Self-hosting mail servers is tricky, same as self-hosting ssh, http, or whatever else. But it's totally doable even on an aging RPi. No, you don’t need to train expensive spam detection because it's enough to have very strict rules on where you get mail from and drop 99% of the traffic because it will be compliant. No, you don’t need to run crowdstrike for a server that accepts bytes and stores them for another server (IMAP) to offer them to you. You don’t even need an antivirus, it's not part of mail hosting, really.

Instead of bickering and posturing, you could have spent your time better educating OP on the best practices, e.g. like this.