You do NOT have to allow MiaB to take over control of your DNS. In fact, I recommend against it strongly if you have a web server elsewhere or any other services on the domain. The guide was written with one goal in mind and that was to create a simple to maintain web server that someone COULD host a static website on if they so chose.
If you want to continue to host DNS elsewhere, such as with your registrar, or cloud provider then these are the steps that you need to take:
Ignore setting the glue records section of the install guide.
Once the box is set up, enter the A record(s) for your other services in the MiaB Custom DNS page. (This step is optional - it just makes the next step easier for some)
Copy the DNS entries from the External DNS page in the admin area to your DNS provider.
Ubuntu 18 introduced unattended-upgrades which MiaB uses. This automatically does the package upgrades, so there is nothing to do but an occasional reboot. You will be notified via the admin page when a reboot is necessary.
Honestly, it is much more secure to set up a sudoer than to use the root account for everything. So yes, I’d recommend that as well as turning off root login via SSH.
I also recommend changing the SSH port from the default port 22.
Yes, indeed it is possible. You would have to modify the install script(s). It can be done if you know your way around the command line and using a text editor. Of course, this is an unsupported modification so the package maintainers and others will be unable to offer support if something goes wrong.
Personally, I am with you. Not for the same reason though. My users simply do not use NextCloud. (Of course, I have never even told them that it is there!)