Make sure to install
Samsung Magician and apply the firmware update to the 990 PRO, first thing. It has an important bugfix.
Normally you always wanna use M2_1 first, because in most cases it gets its PCIe lanes directly from the CPU and has the fastest PCIe generation the CPU supports.
For the other slots, you look at the specs/manual for any conflicts and lower PCIe speeds, but they will all go through the chipset. So when you have a couple of possibilities where you still get the maximum PCIe speed, then you look at the placement of the M.2 slots on the board. You want to avoid slots which get a lot of heat from your GPU.
A lot of GPUs push hot air out mostly to the sides, here's a rather extreme GeForce 4090:
If you look at where the first PCIe slot is, then M2_2 usually bears the brunt of that heat towards the board's side, with M2_3 also catching a bit of it.
So these M.2 slots i would use last. I would use M2_1 first, M2_5 second, and only then go to M2_2 and M2_3. M2_4 is the "worst" one (PCIe 3.0 and SATA_7 conflict).
All the M.2 slots have heatsinks for the SSDs, just make sure you install the SSDs and heatsinks as they say in the manual, and don't forget to peel off the protective film from the thermal pads under the heatsinks.
