I have been experimenting with different sql databases in nextcloud after installing it based on the instructions in the first post.  My recommendation would be to go with Postgres11 instead of MongoDB in general. The main difference is CPU utilisation of the NAS and achievable upload speed. With MongoDB my i3 processor was at 100% utilisation when uploading larger amounts of files. In general the upload was pretty slow and also the web performance of Nextcloud was not great. With MongoDB,