My NAS all-purpose array is currently made up of 4x 2TB 2.5" HDDs, with of course a view to future expansion.
I want to switch to SSDs (SSD for always-on, HDDs for cold backups). I've held off on the transition because I was unsure which direction to go in (2.5" SATA vs m.2 SATA vs m.2 NVMe), and didn't want to make an expensive mistake by sinking money into the wrong solution.

Without knowing the physical build/setup, just in general, which would you say is the right path? (I still want all disks to be 2TB)