I think BT 5.0 includes BT Classic for backwards compatibility. So I would think that you should be able to pair the Apple Watch with your KS10’s. I’ve paired my Apple Watch 6 with my Phone Clip+ via classic BT and streamed a timer alert from the watch through the PC+ to my Quattro’s. You say you want to pair for phone calls so maybe that’s a different animal (my ignorance showing) than pairing for media streaming. I think that you can use an Apple Watch as a podcast player to classic BT earbuds and headphones, for example.
On the Family Setup, that only works with the GPS+Cellular variants of the Apple Watch. (2020 review):
Apple Watch Cellular vs GPS: What’s different and which should you buy? (pocketnow.com)
The following Apple web page delineates what you can do with and without connectivity to your iPhone by any means, including cellular. Apple coyly doesn’t say explicitly, “BTW. These features don’t work without connectivity to your iPhone actually in play…” but by the process of subtraction from what works with connectivity (top group of features) vs. what works with the iPhone not connected by cellular or any other means(bottom group of feattures), if you try to use an Apple Watch, even the GPS+Cellular variant, with no connection to your iPhone, you’ll just be using a comparatively dumbed down device - and if you want to use your Apple Watch in conjunction with your iPhone via cellular even if you’re 1,000 miles away, you’ll have to leave the iPhone at home turned on, running down its charge (or plugged in charging all the time - not good for long-term battery lifespan).
Use your Apple Watch without your iPhone nearby - Apple Support
Edit_Update: Seems that I’ve misinterpreted the top group a bit in the Use your Apple Watch without your iPhone nearby article. The top group of functions just requires Wi-Fi or cellular connectivity for your Apple Watch but unless there is a qualifying footnote such as the #1 item, they don’t require connectivity to an iPhone. But the list of features is basically for built-in Watch/iPhone functionality. Although I can load almost the entire ReSound Smart 3D app on my Apple Watch, I can only control my HA’s through my watch via my Quattro’s. So perhaps the best answer as to what you can do on an Apple Watch independently of an iPhone very much depends on the exact app and functionality that’s important to you. YMMV.