My two cents on Apple Watch 6 and Garmin, Apple Watch is more consistent connected to the SugarMate app which requires a lot of Data when away from WI-FI. It uses the cloud calendar. The Apple complications are sketchy. They often have to be refreshed as they will be showing —-. Running the Dexcom app on the watch is not bad, but after the last Apple update it seems to drop out as often as Dexcom. There is no way to display BG in the exercise mode. Before I got a Tandem pump, I would use the receiver to display BG when I was working out with my trainer via Zoom. But you can’t connect Dexcom to a pump and a receiver.
Now for Garmin, I am using the Garmin widget on a Garmin bicycle computer, but I suspect it is the same on Garmin watches. It sometimes drops out to —, but my big complaint is that it will lag quite a bit, sometimes as much as 10 minutes. This is not really acceptable when doing high aerobic exercise. Something I would like is for the BG readings to be recorded in Garmin Connect just like speed, heart rate, cadence and power. This way you could see possible correlations between power output and BG.
Perhaps other watch brands are better at having current BG readings. I don’t know. I have a Garmin Fenix 3HR in a drawer, because it was not a model that would work with the Widget. I bought the Apple Watch, which I don’t find as good for more than BG. It won’t connect to other sensors, such as a heart rate strap. I find that wrist optical HR to be inaccurate when exercising.
Signing off.
Our Apple Watch keeps the BGs at the flick of the wrist from SugarMate no problem, as long as we have network.
As far as I know, and unless they have recently changed it, Garmin did not connect directly to the Dex, it needed to go through the share server. So if you were in the mountains and did not have internet connectivity, or lost connectivity, you would not get your numbers. That might also explain the delay you are seeing.
You can do that with Training Peaks. Here is a reference link from a few years ago.
Training Peaks also supports showing power for your workout too. So you can see it all on the same graph - power, speed, heart rate, elevation, CGM, cadence.
(The cool thing is, if you Google “CGM data in training peaks”, FUD is the first link that Google shows. 
)
Yeah that’s what I meant to say. My phone has limited data, which is OK mostly, but the service will not allow turning on iCloud for cell data. So when I am out of range of my home WI-FI it drops out, and I use Dexcom app. But as I am retired, it’s only aggravation when I am away from home or riding on the road.
No mountains here, just gently rolling hills that work me out.
“ Training Peaks also supports showing power for your workout too. So you can see it all on the same graph - power, speed, heart rate, elevation, CGM, cadence.
( The cool thing is, if you Google “CGM data in training peaks”, FUD is the first link that Google shows. )”
I’ll look into that, I have the training peaks app but seldom look at it.
OSWear and Samsung 4 watch now seems to be compatible Dexcom app. 3 months ago I had to side load the app on the watch.
Adafruit just added this:
It’s out-of-stock - it wasn’t when I looked at it originally but that’s actually good. It means a whole load of hackers are buying it and some will be T1Ds, or at least people interested in making it work in the T1 context. There was a previous version, so I suspect it is already in use; this version seems to have been cost engineered ($10 cheaper), so it looks geeky, but that isn’t a bad thing.
Looking again still not available for the samsung 4.