iPhone 6 Plus can't be updated to iOS 13 -EQUALS- I can't my update my Apple Watch 3 to watchOS 6

Unless there is a workaround I have not discovered on the interwebs, it appears that because my iPhone 6 Plus can't be updated to iOS 13, I can't my update my Apple Watch 3 to watchOS 6.

Therefore, must I replace my perfectly good iPhone 6 Plus with an iPhone that *can* be updated to iOS 13 (I know which iPhones, btw) to get to my Apple Watch 3 to watchOS 6? OR is there a novel workaround that I have not heard of yet? Like, can I pair my iPhone with someone else's iPhone 8 running iOS 13 and the iPhone app and upgrade my Apple Watch 3 that way? Or some other way. Thanks folks!