Is it possible to leave a part of the roms on the SD-Card and connect to the NAS for the large ISOs? I don't find anything regarding this case.
yes you can.
no as easy as you may think depending on what you want to do.
- use a mount point for one (or several systems) like psx, wii, ...
- use linux overlays to mount your nas over you sdcard and have a merged view : https://www.blaess.fr/christophe/2014/12/14/le-systeme-overlayfs-de-linux-3-18/ (batocera already uses overlayfs for the system itself)