Can the 4 x 1GbE LAN ports be aggregated for higher network throughput, and if so, what is the maximum speed achievable?

Yes, the 4 x RJ-45 1GbE LAN Ports support Link Aggregation for higher network throughput.

What type of SSDs are included in the 24TB storage, and are they user-upgradable?

The 24TB storage includes 12 x 2TB SATA 2.5" SSDs for ultra-fast storage.

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Synology DiskStation DS2422+ NAS Server with Ryzen 2.2GHz CPU, 32GB Memory, 24TB SSD Storage, 4 x 1GbE LAN Port, DSM Operating System

Synology DiskStation DS2422+, made for a variety of server roles, such as iSCSI targets backup, file storage, email servers, and domain controllers. Powerful, high-capacity, scalable storage that fits in any environment. A powerhouse capable of fitting into small and medium business environments with intensive, multi-user workloads.
Ryzen V1500B 4-Core 2.2GHz CPU; 32GB DDR4 ECC SODIMM Memory; 24TB (12 x 2TB) SATA 2.5" SSDs for Ultra-Fast Storage; 4 x RJ-45 1GbE LAN Ports (with Link Aggregation / Failover support); 2 x USB 3.2 Ports; 1 x Expansion Port; Btrfs File System for Advanced LUN iSCSI Service
Ideal for VM and container storage, lightning-fast file sharing, a network video recorder for substantial surveillance operations, and editing high-quality video, with powerful storage management, syncing, surveillance and backup software.
Synology NAS chassis comes in a sealed box.
Hard drives and memory upgrades included separately, NOT installed, installation required.