BLE communication is too slow and crashes on timeout

Try slowing that down.  Maybe wait a full second or more.  Does the characteristic support notify?  If so, use that instead of a read loop.

I think you might be hitting BLE a bit too fast.

And it could be your sensor isn't able to respond as fast as you expect.

Yes, characteristics support notify, and i was trying do it slower. How fast do you think one characteristic can be read with a notification?

I have the second problem here, with timeout.

Often the watch cannot be connected to the device, sometimes the watch does not even display the scan result of the device. 

And it is very common for the watch to disconnect on its own (more precisely, it will get stuck), on the device side, a communication timeout message will pop up.

Are there any requirements for what should be part of the advertising packet for communication with the garmin watch?

I tend to use notifies if I can, as that way "the other end" can control how often it sends data, plus it cuts down on traffic. 

The way I would approach this is slow things down to say every 5 second, and if that works, speed things up a bit until you start having issues

And what about connecting and scanning, do you have any tip why it's slow? and why is there long timeout ? 

I can try RSSI, but I have both devices next to me on my desk. 

After about 8 seconds after the watch is trying to connect, a message pops up on the device that the timeout has been exceeded.

I'm checking if there is error.

The message pops up on your sensor or on the garmin end?

if it's on the sensor, do you have the code and see why that happens?  Or ask someone that works on that end?

On the sensor end. Complete information from BT module (BM71) is connection terminated with err code timeout 

If you don't have the code, as someone that does.

What it almost sounds like is after the pairing, the sensor is expecting something else to happen before the timeout. 

With the pairing starting the timeout and waiting for something else. 

All problems were caused by a bad signal. After increasing the transmission power of the BM71 module, everything returned to normal. Thanks for all your advice jim_m_58.