You always combine solar power with wind power, that already solves more than 90% of the storage problem. In places like Europe the darker winter is usually quite windy.
For example, last Sunday Germany covered more than 100% of its own power load with renewables even though winter is approaching. Only a small part of that was solar power, most electricity was generated by wind turbines: https://www.energy-charts.info/charts/power/chart.htm?l=en&c...
The baby monitor could have its own SD card and webserver and then you provide a smartphone app which uses local network discovery to find the server and talk to it.
In that case no parent needs to know about Synology or even IP addresses.
I think the biggest pain in hosting your own mail server is getting your outbound mail delivered into the mailboxes of the large providers without being marked as spam. Especially if you don't actually send a lot of mail, so you can never really build up a good IP reputation.
That's why I generally recommend a hybrid setup: Host inbound mail completely by yourself so that you have full control, but ship off outbound mail to a trusted relay of a privacy oriented provider. For example https://posteo.de/en doesn't filter any outgoing messages by sender, so you can send mail from your own domains. Your local Postfix can DKIM sign your mail before sending them to Posteo and if you add include:posteo.de to your SPF record all your mail will be DKIM signed, SPF authenticated and coming from a reputable IP, so all your deliverability issues will be gone.
For example, last Sunday Germany covered more than 100% of its own power load with renewables even though winter is approaching. Only a small part of that was solar power, most electricity was generated by wind turbines: https://www.energy-charts.info/charts/power/chart.htm?l=en&c...