Don't drop 2.5" yet. While M.2 SATA SSDs are comparable in performance to 2.5" SSDs in light workloads, they aren't for heavier workloads. It's physical: more NAND chips = more parallelism/interleaving. The number of controller channels used will determine performance too between 2.5" and M.2. More channels = more parallelism/interleaving. Generally, 2.5" uses 8 channels while M.2 may use 4 or 8 depending on the model.
Don't get caught up in M.2 PCIe sequential numbers. In an OS environment, a higher performance SATA SSD will beat most PCIe SSDs. Look at the low queue depth random numbers for PCIe. They aren't there yet. In steady state - real world performance, a 2.5" SSD is still a top choice.
There aren't enough choices for NVMe drives yet. The price is prohibitive.
2.5" offers a capacity advantage too. Currently, 2TB size is available in HDD and SSD and M.2 is only at 512GB.
The ZM/DM has it right, two of each. It's understandable where size constraints dictate more M.2 usage. Where space isn't an issue, 2.5" drives are still the better choice in many usage scenarios.