I think there is some confusion in your understanding. M.2 is the form factor, not the bus type. There are BOTH PCIe NVME and SATA M.2 drives. So you can have a SATA SSD that is still an M.2 drive. And that's the short version. There are actually a few more potential variables that could be involved depending on the type of device, but for desktops that is pretty much the gist.
If you install another PCIe NVME M.2 drive, it is probably going to steal some PCI lanes from the SATA headers anyhow, so regardless of whether you use a 2.5" SATA SSD or an M.2 SSD of either NVME or SATA design, you are probably still going to use the same number of the same lanes anyhow. For the sake of reducing cable management and potentially having a much faster throughput between your existing NVME drive and the new drive, it is probably wiser to go with an NVME M.2 drive if it is supported by your hardware. But either of them will work and you likely won't see that great of a difference in 90% of what you do unless you often perform rather large sequential single file transfers from one drive to the other. Maybe some, again, depending on how it is used but MOST configurations would see similar performance for most tasks with either style.
If aesthetics are important, then that's a point in favor of M.2, whether NVME or SATA.