I will also add that LDAP is rather difficult to manage. Years ago, I worked with OpenLDAP within a small business. It was a pain to get up and running along with keeping up with the management. I tried something similar to MiaB a while back that did include LDAP out of the box. I was trying it out and as soon as I made some changes to the domain, it stopped working and recovery was more complicated than it was worth. Finding MiaB was a huge relief since it kept the user model simple.
Sometimes configuring an application that consumes it isn’t too bad but applications sometimes use different schemas so it’s not a foregone conclusion that you’ll get what you want from it. If you don’t, you’ll need to learn the LDAP query language to do so.
For friends and family or a small company, MiaB’s model is perfect. There are some tools that aren’t too hard to integrate. For example, there is a script that will allow ejabberd to authenticate against nextcloud.
Of course, that doesn’t get to the core of what Eliter really wants. It’s not a shared log-in as much as it is Single Sign On. That’s possible with Kerberos, but that comes with a whole other level of difficulty.
If external authentication and/or user sync are possible, that’s the direction I personally prefer.