Personally I use Nextcloud, but as you want local only, I have used Thunderbird. It’s fine. Cross platform, open source, remote to local sync. Basically, it ticks all the boxes for a calendar/agenda.
You might also take a look at this list: https://github.com/awesome-selfhosted/awesome-selfhosted#calendar--contacts
Depending on what you are trying to do, not necessarily. NextCloud itself doesn’t really care, as far as I know, as long as it’s address doesn’t change. AIO on the other hand is setup in such a way that it needs a resolvable domain name and a valid certificate for https.
This could be done by spinning up your own certificate authority and dns server, but that is a lot of extra work and would be local network access only.
Another way would be to use a free domain and a free certificate from let’s encrypt. The downside here is that the domain authority could yank your domain at any time, for any reason (as happened to all of the free .ml domains recently). At which point your certificate would also stop working resulting in a situation where you may have to nuke and pave.
If you want to be local access only, I would pick an install path other than AIO. If you want to be able to access NextCloud remotely, purchase a domain name.
A VPN, such as TailScale would be considered local network in this situation.
That looks interesting. Looks like they only ship within the EU though.
I currently use Jellyfin to stream my music collection. It’s all stored on my NAS and I can give access to whomever I like. Downside is that the iOS music client, FinAmp, is… not pretty. It’s functional, but not great. I understand the player situation to be a bit better on the Android side.