WatchFace with a BLE device.

Thanks for answer. Something like this I've expected. IDK if it will work for my situation.

Sometimes pairing takes about 20 seconds, I don't know why. 

With the thingy, the scan/pair/read data only takes a few seconds, so it works fine in this case.  It's not a real practical app, as the thingy goes to sleep after 3 minutes if it's not connected, so you have to have the thingy in your pocket and move around to keep it awake.

I have BLE widgets that talk to a pi that easily connect and get data in under 30 seconds.  That's the timeout for a widget on so of the va/venu devices.  With persisting the scan result I can connect to the device in a second or so, as long as I'm in range.

For my case it's better to have as frequently updates as possible :) But I understand the idea. Watchfaces should not drain the battery.

Here is the app I wrote. https://apps.garmin.com/en-US/apps/e769b70b-b5a2-4534-8a7a-a83131b54152 

So I wanted to show Dose rate on a watch face realtime.

Have you considered adding a background process to your device app, maybe turn it into a widget with a background service?

Unfortunately I've not yet tried to write a widget and I don't know what it can and what not. But I will.  

I have weather widgets with a background to update the data and they also have the advantage that you can show the important stuff in a glance view on a device that supports glances.  You don't even have to open the full widget snd can be a single button press away from a watch face. See

forums.garmin.com/.../widget-glances---a-new-way-to-present-your-data

And in a widget, you can request data in the foreground, so you can have the background run every X minutes, and also try a request when the widget is opened, so the widget will always have data even if the foreground request takes time or gets an error