Not sure about linking 2 HAs together, that could get complicated real quick.
If you have your DMZ setup, put HA in that and only allow web access from your main network. Also with camera, there is a feature to allow direct access if you and the camera are on the same network.
As a homelabber and systems admin, I would start with learning how to use Linux on the command line.
It's all well and good looking at docker, but if you don't know how to move about or edit on the command line. Your going to get stuck real quick.
Once your happy doing simple tasks on the command line, I would actually look at running something like wordpress on a pi or VPS. That will help you understand how to connect services together, and cement your basic command line knowledge.
I moved all my domains from Google to OVH, OVH have an API so you can get certs with certbot. You could also use the API to update records for a dynamic home broadband
I have stuff in new releases.io and also GitHub release RSS feeds in nextcloud, I then sit down once a week and see what needs an update. Reboot when required.
I nextcloud news (RSS) for that sort of thing, you can organise feeds into folders and see an unread list. There is even a mobile app to get whats on the server
your running something on port 443 already, if its nginx thats still running kill it. If not, then found out what is running on port 443 and kill it use ss -nlp to find the process name
I use nginx as the internet facing proxy, write my own config and manage it with source control. Also use traefik in docker land with service labels to configure it
Im not sure I understand your issue, DNS is the magic that allows you to goto home.somedomain.com and get a webpage. But you still need to give DNS the location of the server, much like the contacts in your phone.
DDNS is how you have your home connection attached to a DNS record that is updated when the IP changes, see other comments.
Also once you understand how it all hangs together, you can do some really cool magic to make getting to your services easier
Can confirm that zigbee is much better for IOT things, I find the zigbee stuff is quicker to react to events as there is not WiFI setup. Some WiFi devices running on batterys will have lag while they get WiFi link back up.
If you run home assistant, that has a shopping list. We use that, most other things have a full on meal planner that I don't want