Questions about DS918+

No, it didn't, the additional 4G RAM can simply be inserted into the free slot.
Regarding warranty, Synology writes "Do not remove the pre-installed memory module to avoid voiding the product warranty" and " Synology will not provide full product warranty and technical support if a non-Synology memory module is inserted.
memory module is installed that is not from Synology."

So as long as you

Basically, a Synology NAS with the appropriate configuration (e.g. drive) can be compared to a cloud subscription. Note, however, that the speed of access depends strongly on your internet connection at home. I'm currently running a DS414 with 50Mbit/s upload and access to photos is possible but not very exhilarating unless thumbnails are generated beforehand. With a synchronous 1 Gbit/s internet connection,

hello 80401230
if you need data regularly, the access will be faster because the data is read from the ssd-cache. but probably only makes a difference with heavily used nas. with one ssd you can set up a read-cache. with two ssd's a read/write-cache. on the syno-homepage everything is well described.
greetings rené
ps i tested your mentioned ssd from samsung in a smaller size. works fine.

You can cache on a read-only volume with a single m2 drive or read/write with 2 m2 drives. Not for the OS, at least not officially.
Afterwards, if you find the OS slow, you can add more RAM and remove memory compression (16GB recommended).

The only solution is to do link aggregation ( fr.wikipedia.org/wiki/Agrégation_de_liens ). The DS918 does it, you need a compatible switch, a network card with 2 compatible Gigabit ports on the computer and 2 Ethernet cables between each device.
Not to mention the knowledge needed to configure this mess...

no problem with DLNA playback, even with 4K sources... the weak point will be the TV rather than the NAS!