Two parts and one part of it im not 100% sure about as i dont use a Libre or any app.
For me the Garmin ranges are the best fitness watches, i used the Garmin vivoactive3 - its fantastic and i have in the past had Fitbit and a very cheap "fitness watch" of some Chinese brand or other.
It does allow connection to various third party apps, google suggests it can link to Blucon (i can see installable apps to a "dip+ and "spike" apps pn the garmin app store for it but i would do more research on Garmin connectivity.
Depends if your looking for a fitness watch or a smart watch, for more fitness based requirements garmin wins for me - if your more looking for tech on your wrist maybe some one else will be along with a good option for you.