I can't say anything about Gen5 NvMe SSDs. Possibly consult the compatibility list at AsRock and look for something comparable.
The supplied fan is certainly effective enough and usually surprisingly quiet. Nevertheless, I have installed the Alpenföhn Silvretta. This is larger and howls less under load.

Gen5 is only on the top M.2. The heatsink on the M.2 will depend on its size and the CPU cooler, might need some DIY. The bottom M.2 is Gen4 and no heatsink will fit because of the motherboard tray.
+1 for replacing the stock CPU cooler with the Noctua NH-L9a-AM5, very quiet and efficient with a 8700G.

PCIe 5.0 is generally supported (M.2 slot on the top of the mainboard. The slot on the underside only supports PCIe 4.0 without a heat sink). With the standard CPU cooler, space should be fine. It will probably also work with other coolers, as the heatsink of the T700 is not so high at the sides.
I have not tried the standard CPU cooler. However, it is a lot smaller than the Noctua NH-L9a-AM5 I used. Of course, it depends a lot on the CPU. I run the x600 with an 8700G and the fan always remains pleasantly quiet.