When I looked around for CalDAV solutions the last time Nextcloud was the only one that allowed me to share calendars with my SO. Nextcloud isn‘t very taxing on my system because it doesn‘t do anything most of the time.
Do you know about problems reaching the big player mailservers?
Honestly, I don‘t know. I have never had a confirmed case of an email being rejected or classified as spam. There were some cases of not getting an answer to an email. But that could also be explained by shitty customer service.
It is tricky to setup everything correctly if you are trying to do it all on your own but SNM holds your hand for setting up DKIM, SPF and DMARC. That‘s where some people may have problems. Also, forget about setting up a mail server at home with any IP address you get from your internet provider.