What's a practical way to use a bunch of surplus 512GB SSDs?

For straightforward storage, selling the surplus drives and buying a larger SSD may be less hassle. The capacity adds up, but so do the connections, mounting space and cost of the hardware needed to use them all.

If you only need a few more terabytes, start with the free SATA ports in your existing machine. You don't necessarily need a fancy enclosure, though the drives still need power and a secure place to sit.

You can keep them as separate volumes or stripe them into one larger volume. With a striped volume, one failed drive can take out the whole volume, so don't treat it as a safe place for the only copy of anything.

For a larger SATA setup, look at a PCIe SATA controller or a SAS host bus adapter with suitable cables. That can add connections without buying a separate enclosure for every few drives. Check how many ports the card actually provides, and plan for enough power connectors, mounting space and airflow.

At some point the extra hardware may cost more than the drives are worth to you, so price the whole setup before committing.

For the M.2 drives, check whether they're SATA or NVMe first. M.2 describes the shape and connector, not the storage interface, and an adapter has to support the type you have.

Multi-slot PCIe cards exist for NVMe drives, but some depend on the motherboard supporting PCIe bifurcation to see every drive. Check the card and motherboard documentation before buying one. A card for several NVMe drives won't automatically solve the SATA M.2 side.