With propper case airflow it shouldn't go hot enough for it to matter, neither will a heatsink help if that airflow sucks.
ek sell nvme blocks for cooling the drives but if you want it watercooled them i recommend these Bykski blocks which i use for my nvme drives. Blocks are made of tempered glass with a good solid quality and mount.
You only need to cool the controller as cooling the nand is not a good idea nand needs to be hot(ish) to work properly when writes occur so if your mobo comes with a heat transfer plate then that's all you need to use if not then some little stick on copper heat sinks like these https://www.amazon.com/Heatsink-20x...sr_1_13?keywords=copper+heatsink&sr=8-13&th=1 is all you need
This. NAND wear rate increases if its not warm enough while being written to. So rather than cooling them, a bit of heat is useful. One of my Sabrent Rocket 1TB has its metal foil attached and is aimply air cooled, a fan blows air near it. The long term has shown the core maxed at 50C so is adequate for my use. The other has a heatsink that came with the mobo pushing on the the processor only, but still has the metal foil on it to raise NAND temp a little when in use. It helps that heat passes through the foil first, its a good idea to leave it in place when fitting a heatsink.
I installed my new system drive, Samsung 980 Pro, with Be Quiet MC1 cooler. The drive is idling at 44-45C, so I don't think the NAND is suffering from too low temperature. From what I read, the passive cooling elements of rather conservative size aren't dropping the temps that much anyway.