The Synology DS923+ adds PCIe Gen3 x2 for 10GbE expansion

The DS923+ gives you the usual two 1GbE RJ-45 ports out of the box, but Synology also included a PCIe Gen3 x2 slot that can take a 10GbE or 25GbE network card. That x2 lane width is narrower than the x8 slots on the DS1621+ and DS1821+, so your ceiling is lower — but even a 10GbE card through x2 will massively outpace the built-in Gigabit ports for large file transfers.