If I would go with the DS1819+
what would you recommend to choose to put in the PCIe slot?
What gives better benefits the 10GbE or the SSD cache?

I wish you could install both. But because you can not- you need to decide what is more important. You probably will not need NVMe speed cache card unless you run a web server or database server or have large userbase connecting to your NAS. For consumer grade tasks you can get away with regular 2.5insh SATA SSDs. They are still pretty fast and will improve overall performance.

The 10GbE card is very useful if you need very fast backups or data transfer speed. If you link aggregate 4 LAN ports at the back of the NAS- you can still achieve up to 400MB/s theoretical data transfer speeds.

Here is some more info from Synology:

M2D18

Elevate Your Storage Performance To the Next Level

Designed to supercharge your NAS I/O performance, Synology M.2 SSD adapter M2D18 significantly reduces random I/O latency, benefiting applications such as photo indexing, file search, and virtualization environments.

The dual M.2 SSD slots support both SATA and NVMe SSD, meaning higher SSD read/write caching and more primary drive bays reserved for data storage. Attachable heatsink accelerates thermal dispatching of M.2 SSD and assures system stability with excellent performance. Its flexibility in supporting 2280/2260/2242 form factors and meeting various SSD options saves you from vendor lock-in.

E10G18-T2

Achieve outstanding network performance with minimal effort

Enjoy a smooth migration to high-speed 10Gbps Ethernet. E10G18-T2 delivers superior transmission performance with the capability to offload packet processing and free up CPU resources for more tasks. It supports IEEE 802.3an/ab (10GBASE-T/1000BASE-T) and 802.3x with dual RJ-45 connectors, providing a total data rate of 20Gbps using standard CAT6 or better UTP cabling connections. Features such as Link Aggregation, Jumbo Frame, TSO, and TCP/UDP/IP Checksum offloading satisfy most workloads requiring high performance, making E10G18-T2 an ideal and economical solution for 10Gbps networking adoption.

 

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