If you absolutely need full speed, you want NVME SSD in a Thunderbolt enclosure. 40Gb/sec. $Expensive$
There are things you don’t understand. 40GB/sec is not possible from any available external storage array currently being made for home use. Check the specs. 40GBs is theoretically possible with 80 SSDs in RAID 0. This also assumes that you could get that data down 4 lanes... and there’s the rub:
TB3 is limited to 4 lanes of data as I wrote earlier. An X5 or any RAID 0 array tops out around 3000mBs — same as the internal speed of a 2017–2019 iMac (or a 2015 upgraded to a 970 EVO). Some claim speeds of 3500mBs — one twin-blade claims up to 4000mBs RAID 0 but I’ve not seen real world speed that fast and no NVMe 3 blades are spec’d to put out 2000mBs to only 2 lanes. A quad-blade RAID 0 array tops out around 1500mBs and uses four 970 EVO blades to get there. JBOD increases capacity but a twin runs half speed and a quad runs 1/4 speed. Look it up. NVMe 4 blades require PCIe 4 motherboards — they run at NVMe 3 speeds otherwise.
Less expensive is to install the blade inside the iMac where it belongs. The price difference between an X5 and a 970 pays for the labor and puts the change in your pocket. $ave more by installing it yourself. Easy.
2015 and later — use a fast blade like a 970 EVO or a WD Blue 3D. Costs more than the ones below but a lot faster.
2013-2014 — an inexpensive, slow blade like a 660p or P1 is fine. The PCIe 3 x1 bus cannot take advantage of the 970 speed.