Thanks Klojum , in fact I had seen that tutorial.
In my case redundancy of data is more important than speed, and even that guy had "a mac mini for years" as a NAS. I bet I can still have a reasonable performance by using a Pi3 and 2 external HDDs, like here (yes I know that is using a RPi4 too...).
I will take your comment seriously though and try to place this close to the router, so that I'd be able to connect the Pi with the cable at least.
I just found a somewhat different solution here though: It's just using `rsync` to make backups. This setup would allow me to keep the Pi as-is with libreelec I guess (can I assume that rsync IS available? Haven't checked yet but I should assume) - provided I can offer nfs and samba sharing through it. It's not ideal but in some ways involves less change. I will have to decide.