Jasone, what you want to do is 100% supported already by xdrip+. Data can be exported as CSV; data collected on the watch is synced to the phone when in BT range. Any access to the data (other than graphical display) is done through the phone.
The one caveat is that you would be using "wild" software, developed during people's spare time out of interest, without the technical expertise the developers of the actual sensor technology at Dexcom have. So, there are bugs from time to time, and there may be reason for less "faith" in the accuracy/reliability of the software.
Okay, with standard disclaimer, here's my real-world experience: The only bugs I've encountered have been my own, introduced in the last week or so since I started messing with the app myself. These are in my private, local development copy, so anything you'd get from the "official" releases wouldn't have these issues. Also, big plus is that any bugs you find will get interactive attention and depending on the severity fixed quickly.
The app has been for me, so far, more accurate than the Dexcom receiver. Better than the vaunted new 505 firmware that came out a year or so ago. I can get away with calibrating less often than I did with the receiver.
As for the rich functionality, the only way to get a feel for that is to use the app.
Recommendations: First, you really want to go with a G5 system, as the phone and watch can talk directly to transmitter. You can leave the receiver at home as a backup. You can do it with a G4 but you then have to build a small bridge device that has to be carried around too, which nixes the benefit of shedding a device.
As for watch, people seem to be having good results with the Sony Smartwatch. I have a Huawei, which doesn't work with the system, no one knows why. I plan to debug it, but I'm working on a different project right now adding another android App Widget to the application for use with the Wearable Widgets app and Samsung Gear smartwatches (I have an S2). Here's what it looks like at the moment:
Again, however, I'm the only person that has this, and I would recommend an Android Wear watch over the Gear S2 for xdrip+, if that is your primary reason for the smartwatch. When your ready to get going, drop me a message and I'll help!