You might find FileRun or FileStash useful.
Also, about backups, ideally you want to have a regular backup to either an external drive that you disconnect when it's not backing up, or to somewhere that isn't the physical location of your server.
Ask yourself "how much can I afford to loose?" to get an idea as to how frequently you should do a backup.
If they are absolutely adamant that they not use hosted solutions, I'd just suggest paying for a you-provide-hosting service like https://www.filerun.com/buy, change the branding/logo/etc, and charge your client more. Any of the numerous OS products would work as well and the skinning would take substantially less effort than writing one from scratch.
Building your own would be a complete waste of time.
I installed FileRun recently and that might get you close. It's fast and the search is pretty good as it can integrate Apache Tika, I like the OnlyOffice integration as well. It's closed-source, which isn't great for me, but you get 3 accounts without having to pay.
I think I had checked this out before and for myself I found it too limiting but I wouldn't imagine it'd be rough on a Pi.
I'd recommend checking out Filerun because it too has FTP capabilities if you'd like and many other features. Might run a little heavier but it works great for myself.
Filerun would be a great alternative to Nextcloud if you don't need all the other features that come with Nextcloud. Check it out here. Keep in mind that there's a max amount of users (3 I believe but you can email them about this and they can get you more user accounts in your use case). Everytime I used Nextcloud it felt as if it was dying on me, so I just gave up trying to make it work cause it didn't seem worth it. Filerun however, has been rock solid for me all the way through.
If you still want something more lightweight, droppy seems like it would do the job for you. I used that for a while before switching over to Filerun since I needed to support several users. However, droppy doesn't have authentication of any kind and you'd need to use your ldap login to manage that.
Another similar one I recently found out about is surfer. Again, no authentication for this one.
Seafile was great for me until they stopped supporting Windows. My environment is heavily Windows Server and I gave up on Docker Enterprise. My solution was FileRun. It's not free for organizations, but home users can request extra user licenses and I've been very happy with it. The Android app is awful, though.
If you want to use FileRun together with a family website or non-profit organization website, contact us and we'll provide a free license which will cover your entire group of users. (Before applying, please note that it must be obvious that the usage is going to be non-commercial, an existing website must be up (using a registered domain name) and the application cannot be anonymous.)