Went ahead and moved the sim for my only line from the 6s+ over to the Nexus 6P, set up the Huawei Watch and put the iPhone and Apple Watch in the drawer for now.
Trying to keep some detailed notes comparing living with the 2 different sets of hardware in 2 different ecoysystems, will eventually share for anyone who might be interested.
Oh, and FWIW, I shut off iMessage on my iPhone, unlinked the phone number from my other Apple devices, moved the sim to my Nexus, used the
deregister link just in case, sent a text from my both my wife and daughter's iPhones and immediately got the text messages. So for those hesitant to switch because of the iMessage 'handcuff', rest assured that while slightly annoying, take the correct steps (took all of a minute or two) and you shouldn't have any issues. This is probably the 3rd time I've done it over the past few years without issue.