For Nas the ssd is useless only reduces the life span of the ssd ,and for Gigabit-LAN 110mb/s a hard disk is enough,better more ram: page cash ;)

Yes
SSD cache requires SSD ✓
Crucial MX 500 compatible with DS916 ✓
see <https://www.synology.com/de-de...>
1TB is a bit big for the cache in my opinion. If you buy the 500GB or 250GB you could save some more money. (sry Digitec :-))

Thank you very much, I am also planning to get the 500GB and had possibly selected the 1TB. :) Thanks