I agree the phone won't last that long on battery alone. I have a charger in my frunk and keep my phone there. Reception is great for both of my Senas.1. I live in a city with a population of 400,000 and can't count on 4G cellular service at home. Usually 2 bars but sometimes 2G or none. When crossing the entire US, there's a lot of empty areas which have poor coverage (I see this when traveling back home to see family).
2. A few years ago I was part of a forum where a rider went missing on a trip. Had lunch with a friend one day but didn't arrive at the hotel that night (or something like that). They eventually found his body 30 feet or so down a hill but it took nearly two weeks. Not sure if a SPOT would have made a difference but it would have saved suffering time for his family. He was a seasoned distance rider so they assume he tried to avoid a deer or something and lost control.
Other thoughts I haven't seen mentioned...
I'd take and extra pair of boots. If you get caught in a rainstorm, you don't want to be stuck riding in wet boots (mine seem to get wet even with rain gear). Or riding in damp, sweaty boots day after day. Extra gloves in case they get wet in unexpected rain.
As far as preparing, that's not too difficult since you've decided to mostly use hotels and you're retired. Ride 300-350 miles. Come home and sleep in your own bed. Repeat for a week. While doing this, use mostly what you'll take with you to get an idea of what you need (make note of what you don't need). Adjust your trip time accordingly.
In 11 days, weather could be bad enough to delay you. Plan to arrive at least 2 days prior to spending time with your son. If you're delayed a day, you'll still have a day to rest up before time with your son.
I didn't seen any USB ports mentioned. If you're using your phone for GPS and directions, it probably won't last 300 miles on battery alone. Get bluetooth in your helmet so you can hear the directions. Might need a different windshield so you can hear it better. Maybe a phone mount if you want to see the maps but waterproof mounts can make the phone hotter than usual.
In motels it won't be a problem, but if camping I'd say to make sure to carry a battery pack so you can recharge the Sena every night - I don't have to worry about my phone, but the Sena isn't plugged in and I've had it go dead on long days.