OP:
Tell us
what you are going to STORE on your external SSD.
For things like a photo library, music library, movies, you don't really NEED "super high speed". An "ordinary" 2.5" SATA SSD can do the job just fine. They're cheap, and they run the coolest of all. Put one into an enclosure like this:
SABRENT 2.5 Inch SATA to USB 3.0 Tool Free External Hard Drive Enclosure [Optimized for SSD, Support UASP SATA III] Black (EC-UASP)
tinyurl.com
...and you should see read speeds in the 420-430MBps range over USB3a. Maybe a little better using USB3c.
The next "step up" would be to get a USB3.1 gen2 enclosure and then put an nvme blade drive into it.
But be aware that nvme drives "run hotter" than do 2.5" SATA drives.
This will become noticeable under "heavy write" situtations.
I have one of these I put together myself. I used an Orico enclosure and a Crucial nvme blade. Works fine for backups.
These will give you reads in the 960MBps range, but I've noticed that it will "throttle back" under heavy loads and high heat. I sense that this is part of the nvme control technology that will throttle back a drive under heavy write loads and high heat, with the objective of reducing write errors (due to heat). I could be wrong about this presumption.
The fastest speeds are from thunderbolt3 enclosures with nvme blade SSDs.
But they are expensive and again -- due to the higher speeds, they can get hotter than the same drives used in USB3.1 gen2 enclosures. There have been complaints documented over at macintouch.com about severe throttling by the Samsung X5 tbolt3 drive under heavy loads.
Again, much depends on what you'll be using the drive for.
For "general external storage", I'd be fine with a 2.5" drive or USB3.1 gen2.
