Area-51 R5, SSD upgrade options

My current config is 512GB M.2 PCIe NVMe SSD (Boot) + 2TB 7200RPM SATA.

I need at least a 1TB SSD now. From what I understand, I can't add a second M.2 SSD drive because there is only one slot M.2 slot used by the existing SSD drive. So, if I replace my existing SSD I need to clone it to the new one. That doesn't sound like fun...

I also believe I can add a SATA 2.5 SSD to one of the existing, empty 2.5 slots that I have, using one of the available SATA drive connectors (#18 & #19 on the System-board components diagram). Do I need to worry about performance loss here? I am just now learning that there are multiple connectors for SSD drives (I'm a software engineer, I don't have to think hard about hardware lol).

There also appears to be M.2 -> PCIe adapters that I can use if I want to use up one of my PCIe slots instead? But I'm not sure, I'm starting to get lost with all of these options.

Which is the best upgrade path?