I use the tchapi docker image for the caldav server (die to the LDAP support for the user Auth) and davx5 for the android integration.
In Desktop thunderbird already have a native integration and with iPhone is also working fine.
No problems so far in almost a year, they work reliable and smooth. The only point I somehow miss is the lack of push notifications from the server to the devices, but it is not a deal breaker from me
Yes, it will be enough if your services are not exposed via port forwarding , tailscale / zerotier are super convenient for this.
Honestly, if I were you I would start thinking in having a small computer just to act like a proxy / firewall of you synology, or even better, just run the applications on that computer and let the nas only serve files and data.
It is much easier to support, maintain and hardening a debain with a minimal intallation than nay synology box just because the amount of resources available to do so. In this easy way you could extent the life of your nas far beyond the end of life of the Sw