As for monitoring: If you only run it for yourself or a small org, really, you can do just fine without monitoring. If the mail access side breaks, you'll notice, if the inbound side breaks, you'll probably also notice soon enough. But then, setting up a minimal monit that tells you when storage runs low or the MX is down really isn't that difficult either.
So, no, it's not a big project. It's as much of a big project as owning a car, probably less so, if you consider the amount of effort/time required (assuming you have an IT background, obviously).
It’s quite likely that other, unauthorized folks, are touching your mail system more frequently than you with such a lax attitude.
Definitely outsource your email unless you want the experience.
If you want security it seems reasonable to use ProtonMail.com instead of larger cloud providers like Google or Microsoft.
This is complete nonsense. I cannot imagine that there is anything particularly magical that requires something other than regular security updates on any server.
Manual intervention is borne of changing requirements or broken things.
Why should an email server for a handful of users require a lot of manual intervention?
Really, you sound more like someone who has read some FUD marketing material than someone who knows how to write an exploit or has any other serious IT security expertise. I am pretty sure I never had any unauthorized access to my mail server in the last ~ 20 years that I've been doing this. Obviously, my system isn't immnune from exploitation, but neither are Protonmail's, and you are massively exaggerating the risk.
That sounds like you were doing something very fundamentally very wrong.
I definitely don't need to touch my mail server daily, nor any of the mail servers I manage for others. It's just doing its job, security updates get installed automatically, and every few years I need to do a debian upgrade, and possibly deploy some new security/anti-spam features. In between those upgrades, it's running unattended for many months with absolutely no problems. On average, I touch my mail server maybe once a year.