Not nextcloud, but I have several containers set up to use ‘third party’ database - ie, not running inside the container or compose group, sometimes on a second machine. It definitely complicates the setup. I haven’t noticed any performance issues, but my services are low load.

I would think that the db running on the Pi with storage on the Pi could be faster than the db running on the laptop with storage on the Pi. The Pi db would presumably have higher bandwidth to the Pi storage and only have to transfer the selected data over network, where the laptop db has to do all the db sorting, filter, and processing over the network, although I suppose the db would try to keep all the data in RAM. I know there’s limitations to Pi’s internal bus bandwidth, Maybe you can compare the hdparm performance of the NAS RAID vs the NFS mount.