This is an interesting question!
If i recall correctly, WCAG have some great resources and guidelines on this. The biggest frustration for me with External Linking UX is that the designer often forgets the affordance before the link is actually clicked.
Telling me that I clicked an external link is too late. I want to know that if i click on the link, that will be an external experience. It saves me time, clicks, and confusion.
The usual solution is an external link-icon next to the link, or a general info in close proximity about how links in a specific section will behave.
I see no problem with your solution, but it shouldn’t be an excuse to skip informing the user before they even click the link. A clear indication to let the user decide whether to click or not, then the need for 10 seconds could probably be reduced to 5-7. Or just skip the auto logic.
Some clients I have worked with implemented solutions where they skipped the automated timing and instead had a button similar to “I understand and want to proceed” (and a Cancel/Back option of course). Above the button, it was a short info about that “this is an external link and this is not associated with us, Org. XyZ, and our liability ends here, etc. Etc.”.