xsrv VS glauth

Yes, OpenLDAP installed from this ansible role

It's dead easy to use TCP/SSL instead of UDP to forward logs. Example rsyslog config

I use rsyslog for that since it's the default in Debian. Configuring forwarding is very simple, a single file in /etc/rsyslog.d/forwarding.conf [1]. Note that this setup uses TLS to encrypt messages so you need to create the relevant certificates (I use self-signed certs). Unencrypted TCP or UDP is simpler, but less secure.

preseed.cfg contains a few basic options needed to setup the OS [1]

I do this with https://github.com/nodiscc/xsrv/tree/master/roles/openldap + https://github.com/nodiscc/xsrv/tree/master/roles/samba. It's just a standalone samba server + LDAP auth for users, no DC or anything like that, so I'm not sure this is what you're looking for.

This ansible role (check the tasks/ directory) gives a good breakdown of what is needed to setup a basic LDAP server and authentication service

I use samba in standalone mode with or without LDAP auth https://github.com/nodiscc/xsrv/tree/master/roles/samba

I would go with ldap server such as https://github.com/glauth/glauth. As for apps directly configure with ldap or add a proxy such as https://github.com/authelia/authelia

I was thinking about something along the lines of https://github.com/glauth/glauth - but that tool doesn't support the exact use case, I think. Are there any alternatives?

Another project, glauth: https://github.com/glauth/glauth. it's written in Go and has a declarative config--you can define users and groups in yaml. Makes it really easy to do HA etc.