from what i see in the docs only cat-1 and cat-4 modems are compatible or did I miss where we can use basically any modem as long as it snaps in correctly in the m2 slot?
The slot is mini PCIe form factor.
You should compare the mini PCIe pinout of the module with pinout below.
Minimum required pins are VCC, GND, USIM_VDD, USIM _DATA, USIM_CLK, USIM_RST, USB_DM & USB_DP
Details at technical documentation.
Any mini PCIe with the above pinout is compatible with the Base HAT.