Is there a way to move the OEM license to an external SSD from which the device will boot?

If not, in general, are there any microprocessors that make it easy to boot an OS from an SSD via SATA or some other high-speed connection?
You could use an external USB 3.0 SSD, but it won't be as fast as a direct SATA connection to one. What's wrong with the onboard flash? It's relatively fast.

Anyway, to answer your main question, I believe if you just install Windows on an external drive it will pick up the license automatically.

Side note: DFRobot, you have a serious spam issue on your forum.
Hey there. My name is Viking Peggy. I am here to work with you on this problem. Unless your change your motherboard, your license entitlements will not change.

If you had your Windows 10 license activated previously, you got a digital license. You will never need the product key again once you get digital license. Since, you have a digital license, hence when you do clean install, the license information is automatically picked up from Microsoft activation servers. Windows will activate your Windows license from this information.

Make sure the version installed is same as it was previous. For example, if it was Windows 10 Home, you need to install Windows 10 Home only not Windows 10 Pro. If at any point during installation it asks for the key, choose you will do it later on / skip.

Do let me know if you require any further help on this. Will be glad to help you.